Hanging by a Thread Voyages of Discovery

In May 1939, the crew of the submarine USS Squalus

was struck by disaster, deep below the surface of the Atlantic Ocean.

They were trapped on the ocean floor with their air running out and no means of escape...

the latest victims of what the US Navy dubbed the coffin service.

Their fate depended on one man, naval inventor Charles "Swede" Momsen.

Momsen's attempt to rescue the men of the Squalus would become one

of the most celebrated rescue missions in maritime history.

It kick-started a whole new area of underwater technology and revolutionised our understanding

of what can be achieved in the dangerous and alien world deep beneath the waves.

On May 23rd 1939, a prototype American submarine was preparing for a routine dive.

The exercise was taking place 25 kilometres off New Hampshire on the east coast of America.

-Are you ready for diving, crew?

-Aye, sir.

This was the USS Squalus' 19th test dive -

a timed crash dive for use in emergencies.

-First officer, prepare to dive the boat!

-Prepare to dive the boat!

Under the command of Lt Oliver Naquin...

-Dive the boat.

-Dive the boat.

-The Squalus had to dive to periscope depth - 15 metres - in 60 seconds.

-Mark.

-That's one, two and three, OK?

-A series of levers closed the valves that fed air to the diesel engines.

Green signalled that the sub was watertight.

At 7.40am, the dive began.

Mark.

62 seconds.

Well done, gentlemen.

It seemed a textbook dive, but within seconds it went disastrously wrong.

SCREAMING

Unbelievably, water was pouring through the main induction valves in the rear of the sub.

FRANTIC SHOUTING

Main valve's not working!

We've hit bottom, sir.

Somehow, despite the all-clear on the control panel, a valve was open

and hundreds of tonnes of water were pouring in the sub.

They lost control of her and she went down to the bottom.

Now I'm a professional diver and I know what it's like

when things go wrong, but I've benefited from immediate backup.

These men were on their own.

What was going to happen to them?

Bearing in mind that in the previous 20 years worldwide,

22 subs had been lost, along with the lives of over a thousand men.

They didn't call it the coffin service for nothing.

In the 1930s, submariners like the crew of the Squalus were taking their lives in their hands.

Underwater technology was in its infancy and, in the history

of submarines, no crew had ever been rescued from the ocean depths.

The disaster of the Squalus would become a pivotal event that would change underwater safety forever.

Even today, flooding is a danger that terrifies every submariner

and recruits are trained how to react to any breach of their boat.

In this simulator, the sheer force of a wall of water pouring through

at 14 lbs-per-square-inch pressure is a terrifying experience.

This is really hard work

but of course I knew it was going to happen.

For those men on the Squalus it would just have been a sudden, tremendous shock.

Freezing cold water under high pressure.

We're here at the surface, but even at periscope depth

it's twice what the pressure is at the surface

and the water just comes pouring in under ever increasing pressure.

It wasn't long, despite their best efforts, before

the men in the rear of the sub were completely overwhelmed.

Within a few minutes, water was flooding from the rear to the front of the sub.

As men struggled desperately forwards, the crew who had already

made it into the control room faced an agonising decision.

Either wait for their crew mates to come through and risk the whole sub flooding,

or shut the watertight bulkhead doors and condemn them to certain death.

They were ordered to seal the control room.

SCREAMING

26 men died in those first few minutes.

33 survivors were entombed in what was now a watery coffin stranded on the bottom of the ocean.

89-year-old Carl Bryson is the last living survivor from the Squalus.

Carl joined the Navy as a teenager in 1936.

By the summer of 1939, he was a 22-year-old machinist's mate serving aboard his second sub.

He was in the forward battery when the Squalus went down.

I never really thought about dying there, that would never have crossed my mind.

When the water first started to come in, I didn't have time to think about

anything except how to shut the water off.

Everybody said, "What did you think?"

I didn't think anything except how can we stop the water from coming in?

This is the main induction valve - all the water would have come in here.

-Massive volumes of water pouring in this.

-Tremendous volume.

And it went into both engine rooms.

The crew in the forward section of the Squalus had survived the initial flooding,

but now they were trapped with only enough air to survive for 48 hours...

and a new danger was already upon them.

Water was seeping into the forward battery compartment,

threatening to short-circuit the huge batteries that powered the sub's electric motors.

This is the forward battery, of course.

This is the battery hatch.

Luke opened the hatch and the acid was bubbling and the caps

on the batteries were coming out, so the battery was overheating, we were pulling several thousand amps.

As the batteries heated to a critical level, the chief electrician shut off her power.

Another 30 seconds, probably, and we would have had a battery explosion.

Nobody in the battery compartment would have stayed alive,

the people in the control room would have been lost...

somebody may just possibly have made it out of the forward torpedo room. I doubt it.

With no power, there was no heating,

no light and no hope of raising the sub.

For Captain Naquin, it was time to make a harrowing assessment.

Take a roll call. Yes, Sir.

-Bryson!

-Aye, sir.

CALLS NAMES IN ALPHABETICAL ORDER

Aft torpedo, do you copy?

FAINT BUZZING

Aft torpedo, do you copy?

FAINT BUZZING

Forward battery, do you copy?

FAINT BUZZING

Forward torpedo, do you copy?

FAINT BUZZING

With almost half her crew dead, a dwindling air supply, no power and

no way of reaching the surface, this was a submariner's worst nightmare.

Ever since the sinking of the Lusitania in World War I

by a German U-boat, naval commanders knew they needed submarines.

But the early models produced on both sides of the Atlantic, some of which were even powered by steam,

were often a greater danger to their own crews than enemy shipping - they were steel death traps.

Submarine design had moved on by the '30s, but despite the image portrayed in recruitment films,

service under water was still cramped, noisy and highly dangerous.

The men who served in them had a reputation as mavericks, kind of naval pirates.

It's said the admirals of the day saw these crews as expendable.

But despite the dangers, there was pressing reasons

why young Americans of the 1930s signed up for the coffin service.

'Millions of Americans, men, women and children wait in the cold on bread lines, in soup kitchens.'

The Great Depression of 1929 threw America into turmoil.

By 1932, the economy had virtually collapsed.

'..Construction virtually ceases, mills and factories shut down,

'railroads come to a virtual standstill.'

There were 15 million unemployed

and the wealth of the average American had dropped to the level of 25 years earlier.

'..The ranks of the unemployed are to soar...'

But the submarine service provided an escape

from the hunger and uncertainty of the times.

While the rest of the US was gripped by poverty and unemployment, young sailors were guaranteed

roofs over their heads, three square meals a day and a weekly pay packet.

'Here's one place where mess call means all hands on deck to stow cargo

and there's plenty of room in the hold for seconds.

'After this man stows his gear in his new locker, he hangs up a picture of

'his old schoolteacher and makes himself at home in the comfortable barrack accommodation.'

Submariners got an added bonus - an extra 25 or 30 a month in their pay.

It was called submarine pay but this extra cash was actually danger money.

The submarine service was still the riskiest branch of the Navy.

For the crew of the stricken Squalus trapped on the ocean floor, things were going from bad to worse.

MEN SHOUT

In the forward battery of the sub, seawater was reacting with acid

to produce poisonous chlorine gas, which was beginning to spread.

We weren't to the point of gasping or anything like that, but, er...

we could smell chlorine gas and that certainly was an indicator we wanted out.

But there was no way out.

The stricken sub was on the ocean floor at a depth of 74 metres.

Radio communication was impossible that far down and the last message to base had been garbled.

The sub was actually eight kilometres from where base understood her to be.

Well and truly lost.

The crew released a marker buoy and some rocket flares, but the chances of rescue were remote.

Trapped in America's newest submarine, all the men could do was pray.

In the '30s and '40s, subs built here at Portsmouth naval yard were at the forefront of submarine design

and 20,000 men built virtually half of America's submarine fleet for World War II.

Squalus was at the cutting edge of these developments and yet still the sea took her.

Now the race was on to find her, but even if she was found,

the big question remained - could those men be saved?

The answer to that lay with one man, Lieutenant Charles Momsen, nicknamed "Swede" Momsen.

In 1925, 14 years before the Squalus disaster,

Momsen was a sub commander and was badly shaken by the tragic sinking of his vessel's sister submarine.

Stranded on the bottom of the ocean, several of Momsen's friends lost their lives

while the Navy stood by helplessly.

Momsen was determined things had to change.

Submarines had to become safer.

Helen Hart Momsen is Charles Momsen's granddaughter.

Swede Momsen is her hero and she knows his story inside out.

He had lost friends, people he went to the naval academy with had

been lost in submarine disasters, people that he actually knew.

One of the men, when they opened the submarine after they salvaged it,

his fingers were all torn to stubs because he had tried to open the hatch, which would have been

impossible even without the water on top of it, but I guess people just do terrible things in their final hours

and he was just overwhelmed because, at first he thought, "Well, it wouldn't be so bad -

"they probably just went to sleep, they probably just died a simple death."

But when they opened the hatch and he realised the agony they had

gone through, he said, "It can't be this way, it just can't be this way."

The year after the S51 went down, Momsen submitted plans to the Navy's

bureau of construction for a device that could rescue trapped submariners.

Over a year later, he discovered that they hadn't even been opened.

He conceived of the notion of the bell, the rescue chamber and

they just ignored him.

It's always difficult to try and prove a point or make your way when

you're going against the stream or when you're going against the brass

and, of course, back in those days the Navy was more or less run by what they called surface admirals.

They had all served on surface vessels and they weren't sympathetic with the submarine service,

they saw it as a bunch of mavericks and my grandfather was the biggest maverick of all.

Momsen lobbied the bureau to take his ideas on board, but again and again he was turned down.

Then in 1927, another sub, the S4, was lost with all hands.

Determined not to be thwarted by Navy bureaucracy a second time, Momsen began developing rescue ideas

without the knowledge of his commanding officers.

15 years before Cousteau invented the aqualung, Momsen set to work on

something small-scale that he could design and test himself.

A remarkable breathing device that gave submariners a chance of reaching the surface from 100m down.

He had a plan for the Momsen lung and they gathered together pieces of

hose and metal and inner tubes and put together the Momsen lung and then he tested it in a swimming pool

and risked his own life, so it was his own money, his own life, his own time.

'The lung resembles and works in rough principle like a gas mask.

'Air exhaled into the device passes through soda lime which

'removes the waste carbon dioxide and replaces it with fresh oxygen.

'When each student has mastered the use of the lung,

'he is then ready for the first attempt at underwater breathing.

'The preliminary ascent is made from a very shallow level.'

Crikey, it looks like a hot-water bottle.

Doesn't it? It does. It does.

OK, how does it work?

-I met a man who actually was saved with this from the Tang.

-This one?

-Yeah, out in the Formosa Straits.

-Oh, I'd better be careful with this.

He ascended from a submarine with this.

And this is what goes in your mouth.

Right, that looks like a modern-day...

-Right.

-..regulator mouthpiece.

Hold that up.

'Charging their lungs with oxygen, the men pass up through the escape hatch

'one at a time, holding securely to the

'marker line and taking particular care to pause at the designated intervals for decompression.'

The Momsen lung was the first truly successful underwater breathing device for a submariner.

Filled with pure oxygen that recycled during breathing, it didn't allow you to stay

under water for long, but it could save the life of a stranded sailor.

'This man has safely reached the surface from a depth of 100 feet.'

Wow. You've got the same pressure as on me, the water pushing on here, so equal pressure.

-Right.

-Fantastic. It just seems incredibly simple.

-Yeah, it does.

-I mean there's no diving gear, no diving suit.

-No.

Get that escape-hatch pressure equalised.

-Mmm.

-Open it, put this in, goggles on, make a run for it.

-Right.

Wow.

'At the submarine base in Pearl Harbor...'

This time the top brass couldn't ignore Momsen and, begrudgingly, they came round to his idea.

'Under the supervision of Admiral Momsen, inventor of the famous Momsen lung, the future submariners

-'are ready for the 100-foot tower which holds...'

-The Navy adopted the Momsen lung, as it became known.

Thousands were ordered to equip every sub in the fleet.

Floyd Matthews worked with Momsen, training submariners to use the lung.

He's now 103.

That's 100 feet, you know, we had three different positions -

the bottom - that's 100 feet, one at 18 feet, one at 50.

You see, we gradually worked them up to 100 feet.

-You could do 100 foot, no problem?

-Oh, yeah.

I could jump overboard and go along the bottom.

Yeah.

You exhale and you just keep on going down but you've got

to have something to breathe when you get there, though.

You're empty.

Yeah.

So what are your memories of Momsen?

He was an innovator, you know.

The man was just nothing less than a genius

and he could do anything, just about, yes.

The Squalus was equipped with Momsen lungs for all its crew.

The men had been trained how to use them, but Captain Naquin was deeply concerned.

The Atlantic was freezing cold and the chances of getting all 33 men out were remote.

We had planned an escape using a Momsen lung.

We had the grease, we had the lungs, the water was cold, of course,

and we were going to grease down and the captain had selected Greek Medeiros to be the first man out to

let the buoy out with the line on it because we had to have a line to keep us from shooting up to the surface.

We could have gotten...the first group could have gotten out of the boat, no question, but whether he

could keep going or not, that was questionable.

Anyhow, we had used up a lot of oxygen and the old man decided that

it was safer to wait than it was to try and escape, so...he decided to wait.

But Captain Naquin had no idea that his last location radioed back to base had been garbled.

They were lost.

With the Squalus now out of radio contact for several hours, a second submarine from the

Portsmouth navy yard had been sent to her last reported position...

..not realising it was looking in the wrong place.

There was no sign of the Squalus' marker buoy and no trace of any flares.

The search dragged on and on.

In the sub below, cold and hunger were taking hold.

With surface contact long overdue, tinned food was given out to keep up morale.

Pineapple seemed to be a favourite.

Man, it was cold.

Every place that you had condensation in the torpedo room from your breathing...

a skim of ice.

But cold and hunger weren't the only dangers.

With every breath they took, the men were using up vital oxygen,

and with each passing hour, the chances of survival became ever slimmer.

Yet four hours after the Squalus went missing, nobody even knew where she was.

Off the coast of New Hampshire, Squalus' sister ship, the Sculpin,

was desperately searching for the downed sub.

Finally at 12.40 on the 23rd May, the lookout spotted Squalus' marker buoy

and inside was the telephone connected to the submarine.

NAQUIN: 'This is the USS Squalus, over.'

This is the USS Squalus.

Is there anybody up there? Over.

'USS Sculpin. Are you receiving?

-'Over.'

-Yes...

Hello?

This is the USS Squalus.

Just as Captain Naquin said a few words, a big swell came up

and broke the cable.

Communication was lost with the submarine,

but the men on the top did know that some of those men were alive.

They could do nothing to help them and the whole world was watching.

'May 23rd 1939, the submarine Squalus lies on the ocean bottom off Portsmouth, New Hampshire.

'59 men are trapped inside...'

As a fleet assembled above the Squalus, the world knew there were survivors below.

Papers rushed to print the story.

The plight of the crew became front-page news.

The pressure was on to do something for the men,

but so far no crew had even been rescued from a sunken sub - it was simply too difficult a challenge.

For wives and families waiting in the town of Portsmouth, it was an agonising time.

The whole of the town here at Portsmouth was looking at the navy base over there for answers.

The whole town would have just been waiting and hoping for news.

In the sub itself, the precious air was becoming fouler by the minute.

Each time the men breathed out, oxygen was being replaced with

poisonous carbon dioxide which had to be mopped up with soda lime.

In desperation, the Navy at last turned to the man they had once ignored...Charles Momsen.

Still unrelenting in his drive to improve safety,

he was quietly tucked away in research and development.

Following his success with the lung, Momsen had dusted off his plans for

the rescue chamber, which had originally been scorned by the top brass.

This is a submarine rescue bell based on Momsen's design.

It's an incredible simple bit of kit, hardly any moving parts at all.

It was counted on as being something that would rescue 30-plus men.

And yet it was just completely unproven.

Just countless gauges and valves in here.

Some of them are to control buoyancy,

some are to control winches,

and I can't believe you'd get two operators in here and up

to seven rescued men - there's absolutely no room whatsoever.

The trick to it is sending it down to the bottom

and accurately locating it over the submarine escape hatch,

and the key to that is this thing here.

There's a rubber gasket under there and that provides a perfect seal over the hatch.

When this reaches the submarine,

the water is blown out of it and the water pressure itself,

which at the Squalus' depth was about 120 lbs per square inch,

pushes this on to the submarine and squeezes it in place.

It provides a perfect seal, the men can open up the hatch from inside and enter into here.

The problem, though, is if this isn't sat absolutely level,

you get an imperfect seal, the whole of the sea can just rush in - complete disaster.

'Rescue vessels, led by the Falcon, locate the sub and prepare to send down a newly developed rescue bell.'

It was Momsen's big moment.

With 33 lives at stake and time running out, the Navy had

to take a chance with the maverick inventor and his innovative chamber.

'Never before has a diving bell like that been used for actual rescue.

'Will it work, and at that depth?

'Aboard the rescue fleet, they can only hope.'

For the chamber seal to work, it had to fit precisely over the sub's escape hatch.

This meant a diver had to go down first to attach a guide cable to the hatch handle.

The divers who took on this challenge were the astronauts of their day.

Tough and determined, they risked their lives to push the

boundaries of human knowledge, with only the most primitive equipment.

And this is the kind of kit they had to use.

It's called hard-hat gear and it's very heavy and cumbersome.

One boot alone, this weighs about ten kilos.

So some real problems with it. Firstly, you can only dive very, very close to the ship

because the diver is lowered down from the boat on the surface.

Secondly, they're pulling a long air hose behind them and that air hose in Momsen's case

would have been 75 metres long so it would have weighed a ton, making the dive almost impossible.

But it wasn't just the diving gear that was primitive.

At the time, we only had a very basic understanding of how our bodies react to being at pressure.

So Momsen dedicated himself to learning how that happened

and he developed diving tanks, just like this one used by the Royal Navy here at Gosport.

What Momsen and his team were beginning to discover was that as a diver descends, water pressure

squeezes nitrogen from the air being breathed into a diver's bloodstream and body tissues.

At high pressure, like there are right here,

at 30 metres, at high pressure, this nitrogen affects our thinking.

It's a very pleasant feeling, let me tell you,

but it can lead to problems because it feels like a mildly drunken state

and it means that, as pleasant as it feels to me, and I've had it,

and I guess I must be experiencing it right now,

it means that on a deep work dive, it could lead to fatal mistakes.

24 hours after the Squalus went down, the first diver was ready

to be lowered into the freezing waters of the North Atlantic.

'Deep in the sea there, 33 men are alive, in danger of dying for lack of air.

'"Get the living out!" is the cry as down goes the diver.'

The diver was Martin Sibitzky.

His task was crucial to the rescue - he had to fasten the cable which

would guide the bell down to the sub's escape hatch.

With the concentration of carbon dioxide rising with every breath,

the air in the sub was becoming more poisonous by the minute.

Sibitzky had to succeed, and quickly.

On this mission, there was no room for error.

When Sibitzky got down there and started to work really hard

dragging that heavy cable around, he was breathing more air, which meant that he got nitrogen narcosis.

He became physically fatigued and very confused, almost drunk.

The rescue was on the verge of collapse when, back on deck, Momsen stepped in.

Momsen knew exactly what Sibitzky was going through, so he talked him through it, step by careful step.

Momsen helped Sibitzky gather his thoughts and overcome the effects of nitrogen narcosis.

At last he was able to clip the cable on.

The first stage of the operation was complete...

but the hardest part was still to come.

Now it was time for Momsen's chamber to be put to the test.

'The crew of the rescue chamber climb in for their risky adventure.

'The idea is to lower it onto the sunken sub, make it fast to a hatch,

'open the hatch and bring the survivors up into the rescue chamber.

'So beneath the surface it sinks, for life saving without precedent.

'This occurs a little more than 24 hours

'after the US Submarine Squalus sank while making a practice dive off Portsmouth.'

Lowered by a support cable, the chamber began its descent.

Though it had never been tested in a real rescue situation,

it was the only hope for the men in the Squalus.

But would it work? Would the seal hold?

At 12 noon, the chamber landed over the escape hatch.

The seal held.

Yeah!

30 hours after the Squalus first hit the bottom, the unbelievable had happened.

A rescue mission had reached the submarine.

Carl Bryson watched the first eight men get into the bell.

They were the crew members most affected by the cold and poor air.

It was essential to get the weakest to the surface first.

No-one knew how long the rescue would last, if the weather would hold

or indeed if the bell could actually manage the four journeys needed to lift the survivors to safety.

Under Momsen's orders, the bell was raised carrying the first survivors.

Valves let in air to the ballast tanks,

and inch by inch, the chamber rose...

..guided by the cable to the ship above.

For now, everything seemed to be working perfectly.

At last, the bell made it to the surface.

'There is it bubbling and breaking the water,

'the dramatic sight, the sudden appearance of the diving bell.

'All the rescue power of the Navy mobilised and here is the climax,

'the rescue chamber coming up from its first descent.

'Hoisted up. What's in it?

'There are anxious wives and family waiting tensely.

'Open it up and then out they climb, survivors, the first one.

'So weak he has to be helped after being entombed for 24 hours at the bottom of the sea.

'One after the other, seven in all are brought up in this first trip of the rescue chamber.'

It was a historic moment.

For the first time ever, men had been rescued from a submarine on the sea floor

and in that instant, everything Momsen had worked for was validated.

But it was far from over. There were still 25 men to be brought up.

The sub was freezing and the air was getting fouler by the minute.

There was no time to waste.

The next two dives went without a hitch, with 18 more men being brought up safely to the surface.

The chamber was sent back down to the Squalus for the final time, a little before 8pm.

For Carl Bryson and the last few survivors, struggling against rising carbon dioxide

and the constant threat of chlorine gas, it seemed to be the end of their ordeal.

So we were all up there and...waiting.

Seemed like it took hours.

Man, it was cold and the air was horrible.

It was getting worse all the time.

How did it feel getting in that bell?

Well, it felt good to get in the bell, but when it jammed, it didn't feel so good.

The bell had only risen about ten metres when it stuck fast.

The main cable running down from the bell to the sub below had jammed.

Diver Walter Squire was sent down into the water to free the stuck cable.

He made his way down tentatively.

Squire fumbled around for the cable a few metres below the bell,

he tried to free it but it wouldn't budge.

So on Momsen's orders, he cut it.

Now the full weight of the nine-and-a-half-tonne chamber was

hanging from a single support cable running to the ship above.

Just when it seemed the worst was over,

the diver returning to the surface noticed something disastrous.

The cable left holding the chamber had begun to unravel and snap

and the bell was now dangling from a last single strand.

The men's lives were literally hanging by a thread.

Afraid that this last strand would break, Momsen had to order the chamber gently lowered back down to

the sea bed, so just moments from triumph, the rescue had stalled.

Momsen had them drop us back down in the mud, we were up to 150 foot level then.

They dropped us back down because if that cable had parted

and the exhaust cable and the air cable, then we would be lost.

With the last of the survivors trapped inside the bell, Momsen came up with an all-or-nothing plan.

It was highly risky, but it was their only hope.

He reckoned if the operators inside the bell could carefully open the

valves and blow more compressed air in, they could control its buoyancy,

and his gamble was that they could make it weightless - neither rising nor sinking.

If it worked, it would be light enough that it could be carefully hauled up, hand over hand.

If it didn't, the cable would break and the men would be lost.

He told McDonald to blow for ten seconds, you know.

So McDonald blow the lower compartment.

Then blow 20 seconds...

Then he blow 10 seconds.

Finally, they pulled us clear of the mud

and they had all these people up on the deck, pulling this thing by hand and we got up to about 150 feet and

we went right to the surface like that.

'One of the greatest rescues in the annals of the sea, men saved

'from the sunken submarine Squalus off Portsmouth, New Hampshire.

'Every one of the living brought out alive.

'In the history of the sea, a sunken submarine represents the depth of terror and horror.

'This rescue represents a height of glory.'

We got to the surface...

and I was frozen, man, oh, man.

I tell you, we were lucky, really lucky.

And we had the right people in the right place at the right time.

That makes the difference.

Momsen had done it, he'd saved the lives of 33 men.

The rescue of the crew from the Squalus showed for the first time

that something really could be done for men trapped on the ocean floor.

It was a pivotal moment in the history of undersea exploration.

The Squalus rescue, carried out under the glare of the world's press, had put submarine

safety firmly on the agenda, and within six months of the Squalus sinking, the US Navy

had offered the diving bell plans to 13 other countries in a bid to make submarines safer round the world.

As a proven success, it was adopted by other navies.

And even today, a version of the rescue chamber is still in use.

It's called the McCann chamber.

Named after Momsen's successor in the development programme.

Thanks, mate. I'm here, courtesy of the Italian Navy,

to take part in a submarine rescue training exercise and I'm going to go down here in a McCann bell, which

is essentially the same piece of kit that was used in the Squalus rescue,

to go down to a submarine at 40 metres,

to see what it was really like for those rescuers and for those men from the Squalus.

Thanks very much.

Thank you.

INDISTINCT SPEECH

Very good.

HE SPEAKS ITALIAN

I can feel the pressure

increasing now.

That's because the water is coming in the lower part of the bell

and squeezing all the air into this part of the chamber.

Yeah.

The bell is noisy, it looks primitive,

the air pressure varies wildly as you go up and down, but it works.

The Italians see it almost as an elevator that can run

back and forth from the surface to the sea bed.

We've just landed on a submarine!

It's fantastic to think that a design which is essentially from the '30s is still used today.

And it's not just the Italians.

The Turks, Indians and the US still use essentially the same design

as Momsen's original bell that triumphed in the Squalus rescue.

It's really surreal, actually, because there's the submarine,

that's the top of the submarine,

we're 40 metres in the bottom of the sea.

It's just amazing... going down to a submarine

whilst on the bottom of the sea.

Oh, wow.

Hey, thanks for this, guys.

Thanks very much.

Good Italian espresso served at 40 metres on the bottom of the Med.

I'm going to remember this next time I'm scuba diving at 40 metres and freezing.

Dry, good coffee, good company...

Here we go.

The Squalus rescue was a turning point in the development of underwater technology.

New devices were pioneered that led to some remarkable equipment like this one-atmosphere diving suit.

In one of these, a diver can work hundreds of metres down on the ocean floor, allowing the construction

and maintenance of many of today's most ambitious engineering projects,

like North Sea oil platforms and the undersea pipelines leading from them

which run hundreds of miles along the ocean floor.

These developments would have seemed impossible before Momsen's triumph.

After the Squalus rescue, Momsen was promoted to Commander and his prestige in the Navy just rocketed.

He used his influence to launch a whole new era

of underwater technology and he became the father of modern diving.

One of his most significant contributions was the development of new mixed gasses for deep diving.

By replacing the nitrogen in the air with helium, he completely eliminated nitrogen narcosis.

That meant that professional divers like myself can dive deeper, we can

have shorter decompression times and underwater work is just safer.

In the '60s, Momsen's son, also called Charles, was a real chip off the old block.

He carried on the family tradition by developing the mini-sub Alvin,

seen here looking for a hydrogen bomb lost at sea after a mid-air collision involving a B52 bomber.

Since Alvin was first designed, mini-subs have become lighter

and more manoeuvrable, with ever more specialised functions.

Nowadays they're used all over the world, both by navies and civilian contractors.

As well as submarine rescue and training, they're used for things like investigating wrecks, searching

for lost aircraft, inspecting marine structures and even filming the secret habits of deep-sea creatures.

And one man's vision of what was possible beneath the sea helped pave the way for technology like this.

Momsen was a true pioneer.

He revealed to the world that hugely complex diving operations can take place deep below the surface.

The rescue of the Squalus gave people confidence as they dived ever deeper into this alien world.

Momsen gave hope.

So should things go wrong down here in the abyss, we know that help can be on its way.

After a lifetime dedicated to the safety of men at sea, Momsen died in 1967.

In 2004, the Navy paid him its highest honour and named a destroyer after him.

As for the Squalus itself, it was salvaged from the deep in the months after it sank.

Recommissioned as the Sailfish, it fought through the Second World War.

Her conning tower is still preserved at Portsmouth Navy Yard

as a lasting tribute to the men who served on her.

How does it feel to be on here now, Carl?

Well, brings back a lot of memories.

I lost some very close friends on this boat.

It...

It was a sad thing, it was a heavy price to pay.

Do you think it gave you a unique perspective on life itself?

Oh, yes, oh, yes. Well...

let's say I was always lucky.

I was lucky since then.

I was very lucky, I married a wonderful woman and I got

three wonderful children and six wonderful grandchildren.

Can't get much luckier than that!

If Swede Momsen was here today, what would you say to him?

Thank you, Swede.

You betcha.

I...I couldn't emphasise my gratitude enough, believe me.

I have a medication that I take in my room

and I have a picture of Swede about so big up on my bookcase

and when I take the medication I always say, "Thank you, God,"

and "Thank you, Swede."