0:00:26 > 0:00:30It's still the middle of summer, but just beyond Carrickfergus
0:00:30 > 0:00:32a year-round industry
0:00:32 > 0:00:35is busy stockpiling for the winter.
0:00:37 > 0:00:43Alice Roberts is about to venture into an underground world that's never been filmed before.
0:00:43 > 0:00:46If you're driving along on an icy winter's night
0:00:46 > 0:00:49and your car's not skidding, it's probably because
0:00:49 > 0:00:51the gritter lorries have been out.
0:00:51 > 0:00:55The rock salt could have come from here - on the coast of Northern Ireland.
0:00:55 > 0:01:00'Half a million tons of rock salt are shipped from this little jetty every year.
0:01:00 > 0:01:04'This corner of Ireland sits on top of huge deposits of subterranean salt
0:01:04 > 0:01:09'that stretch all the way across Europe to Russia's infamous salt mines.
0:01:11 > 0:01:15'I don't know quite what I expected from a salt mine,
0:01:15 > 0:01:21- 'but what I never imagined was being able to drive underground.' - This was driven in 1965.
0:01:21 > 0:01:26'Our guide is Jason Hopps the mine surveyor and, yes, salt of the earth.'
0:01:26 > 0:01:31- So how deep does this go down? - The maximum depth in the mine
0:01:31 > 0:01:34is 1,150 feet. This is us just entering the salt now.
0:01:34 > 0:01:37- This is all salt crystals?- Yeah.
0:01:40 > 0:01:45We're coming into quite a big cavern. 'There's over 30 miles of tunnels,
0:01:45 > 0:01:49'yet only 40% of the rock salt in any area is extracted.
0:01:49 > 0:01:53'The rest is left as pillars to shore up the workings.
0:01:54 > 0:01:57'The scale of these man-made caverns is amazing.
0:01:57 > 0:02:01'Even the largest of the excavation vehicles seem dwarfed.
0:02:01 > 0:02:05'Some of the trucks are up to 40 years old,
0:02:05 > 0:02:09'but although the atmosphere is salty, it's also extremely dry,
0:02:09 > 0:02:12'so they hardly rust at all.'
0:02:12 > 0:02:18It's really strange. It's like walking onto the set of a James Bond movie, isn't it?
0:02:18 > 0:02:22It's bizarre. How is rock salt actually formed to begin with?
0:02:22 > 0:02:27Why is there this seam of salt 800 feet under the surface?
0:02:27 > 0:02:33It's basically an old landlocked sea that has evaporated and left the salt behind.
0:02:33 > 0:02:36It's happened, in total, five times in this particular area.
0:02:36 > 0:02:38We've got a full succession of five salt beds.
0:02:38 > 0:02:42At the minute, we're in the fourth deepest, so there's three above us.
0:02:42 > 0:02:48- So several sort of evaporated sea beds have been laid down, one on top of another?- Yeah.
0:02:48 > 0:02:53Although we call it "rock salt", it is sea salt? It's just sea salt that's got trapped in rock?
0:02:53 > 0:02:56It's sea salt with certain other trace elements.
0:02:56 > 0:02:59SIREN WAILS
0:02:59 > 0:03:03'We still have to drive down another 300 feet to reach the faces that are
0:03:03 > 0:03:08'currently being worked - a full 1,150 feet below ground.
0:03:08 > 0:03:10'I've been wondering where everyone is!
0:03:13 > 0:03:17'The rock salt is attacked from two directions.
0:03:18 > 0:03:25'First, it's undermined with a gigantic cutting blade that takes a ten-foot-deep slice from underneath.
0:03:28 > 0:03:32'Then holes are drilled above, ready for explosive charges
0:03:32 > 0:03:35'to be inserted deep inside the rock.'
0:03:35 > 0:03:38- Can we go a bit closer? - Yes, we can go down and see.
0:03:42 > 0:03:44- This is an undercut.- Right.
0:03:44 > 0:03:49That advances in ten feet, which is the same length as your drill holes.
0:03:49 > 0:03:51So this is one of the drill holes
0:03:51 > 0:03:55- where you put the explosives in?- Yes. We pack the explosives in there.
0:03:55 > 0:03:59That drill hole's ten-feet-deep by 50-feet-wide by 20-feet-high.
0:03:59 > 0:04:03It gives us a full face of 600 tons.
0:04:03 > 0:04:07- Really?- Yeah.- So when the explosives are stuck in here and they go off,
0:04:07 > 0:04:11- 600 tons of rock salt falls to the ground?- 600 tons, yes.
0:04:11 > 0:04:14SIREN WAILS
0:04:14 > 0:04:16'Time to withdraw to a safe distance, I think.'
0:04:20 > 0:04:22EXPLOSION
0:04:31 > 0:04:34So this is the last stage of the process?
0:04:34 > 0:04:36That's been blasted off, hasn't it, that rock?
0:04:36 > 0:04:40- Yes. That will have been blasted last night.- Right.
0:04:40 > 0:04:44It'll be taken up to either the crusher or an underground stockpile.
0:04:44 > 0:04:46Right.
0:04:46 > 0:04:49And where does most of the rock salt from this mine end up?
0:04:49 > 0:04:5420% of it or so will end up on Northern Irish and Irish roads.
0:04:54 > 0:04:58- Right.- 50 or 60 will go to either England or Scotland,
0:04:58 > 0:05:03- and then another maybe 20% to the east coast of the United States. - Oh, really?- Yeah.
0:05:07 > 0:05:09A little salt can go a long way.
0:05:09 > 0:05:13Next time you're snowed in, take a good look at that gritter up ahead.
0:05:13 > 0:05:15Chances are that's not any old salt.
0:05:15 > 0:05:18It's actually 250 million years old
0:05:18 > 0:05:23and comes from 1,000 feet under the Northern Irish coast.
0:05:28 > 0:05:33- SEABIRDS SQUAWK - The further west you go, the wilder this coast gets.
0:05:35 > 0:05:38This is a landscape that encourages mavericks.
0:05:38 > 0:05:41But it's not an Irishman who stands out from the pack -
0:05:41 > 0:05:48it's a Cornishman. An eccentric artist who built his fantasy home out of what he found all around him.
0:05:48 > 0:05:51Well, this is what I've come to see - Bendhu House.
0:05:51 > 0:05:55Perched on the cliff-top, it looks a bit like a Second World War fort,
0:05:55 > 0:05:57but it is actually somebody's house.
0:06:02 > 0:06:07'It was in 1936 that Newton Penprase, a Cornish artist,
0:06:07 > 0:06:12'first had his dream to build a house to match his vision of this coastline.
0:06:12 > 0:06:17'For the next 40 years, he worked almost single-handedly to achieve it.
0:06:17 > 0:06:21'Michael and Lorna Ferguson live here now,
0:06:21 > 0:06:25'but their first impressions were rather like mine.'
0:06:25 > 0:06:31- It's a very strange house.- Yes. This is the way I remember the house when I'd pass it as a child.
0:06:31 > 0:06:33I was fascinated - "What's going on here?
0:06:33 > 0:06:36"What is this man building?"
0:06:36 > 0:06:39So you'd seen this house as a child and you ended up living in it?
0:06:39 > 0:06:42Well, I didn't dream I would ever be living in it.
0:06:42 > 0:06:45Nor, at the time, had I any wish to live in it.
0:06:47 > 0:06:53Many people loathed Penprase's unconventional design, not least the planning authorities.
0:06:53 > 0:06:58But he persevered, using whatever materials came to hand, all picked from the seashore.
0:06:59 > 0:07:03In the drawings that he got approved, it says, "all in concrete".
0:07:03 > 0:07:07- Right.- So everything was from the beach initially. He washed the sand
0:07:07 > 0:07:09from the water that came down the cliff,
0:07:09 > 0:07:14and most of the cement was carried down on his shoulder, down the harbour road.
0:07:14 > 0:07:18The bricks were made out of gravel and sand
0:07:18 > 0:07:22and he put a lot of extra windows in during the process as well.
0:07:22 > 0:07:25'In all, Penprase put in no fewer than 50 windows,
0:07:25 > 0:07:29'making the most of Bendhu's panoramic views,
0:07:29 > 0:07:32'from Scotland in the east to Donegal in the west.
0:07:32 > 0:07:36'When the artist died in 1978, the house was still unfinished,
0:07:36 > 0:07:38'but Michael and Lorna have completed his dream.'
0:07:38 > 0:07:42That really is a fantastic view, isn't it?
0:07:42 > 0:07:44Well, Alice, this is the room that we added on.
0:07:44 > 0:07:47The rest is Penprase, but this is what we interpreted
0:07:47 > 0:07:50he would have liked us to do with this part of the house.
0:07:50 > 0:07:53So if you want to come on through with us...
0:07:53 > 0:07:55Right, let's head down below.
0:07:59 > 0:08:04Alice, this is the Zodiac room and you'll see why when you look up at the ceiling.
0:08:04 > 0:08:05That's amazing.
0:08:05 > 0:08:11These are canvases that Penprase painted. He always invited ladies to lie down in the bed,
0:08:11 > 0:08:16- to look up, and he would explain all the Zodiac signs. - Really(?) Right, I see!
0:08:16 > 0:08:19It's almost like you're living in an art installation.
0:08:19 > 0:08:26Yes, I think that it is. Hopefully, what we've added, Penprase would approve of.
0:08:26 > 0:08:31- Would you ever sell it on?- Oh, no, it's become part of our life.
0:08:31 > 0:08:36- What do you think, Michael? - I'd have to finish it first, but I don't think we'll ever move.
0:08:40 > 0:08:43Bendhu House is now a listed building.
0:08:43 > 0:08:48Proof that true individuals, like Newton Penprase, can still have the last laugh.
0:08:51 > 0:08:54Subtitles by Red Bee Media Ltd E-mail subtitling@bbc.co.uk