0:00:27 > 0:00:32It's a part of the coastline where one natural phenomenon dominates pretty much everything,
0:00:32 > 0:00:35from defence to building regs.
0:00:35 > 0:00:39The Welsh coastline has one of the most extreme tidal ranges in the world.
0:00:41 > 0:00:44The extreme tides mean that twice a day, every day,
0:00:44 > 0:00:49there's a vertical rise in sea level of between 12 and 14 metres.
0:00:49 > 0:00:55That's an incredible 40 feet in old money, pretty much as high as that house over there.
0:00:57 > 0:00:59And from time to time,
0:00:59 > 0:01:04those tides create an awesome, almost unbelievable spectacle.
0:01:09 > 0:01:13I'm standing on the banks of the River Severn.
0:01:13 > 0:01:14It's the longest river in Britain,
0:01:14 > 0:01:17much of it marking a border between England and Wales.
0:01:17 > 0:01:19And although it's late at night,
0:01:19 > 0:01:25there are people like me dotted along the river banks, anxiously watching, anxiously waiting to see
0:01:25 > 0:01:28an incredible natural spectacle.
0:01:28 > 0:01:32It's the Severn Bore. I've waited much of my life to see the Severn Bore.
0:01:32 > 0:01:37It's one of nature's miracles, rather like the Northern Lights or a double rainbow.
0:01:37 > 0:01:42If you ask the locals, they'll tell you the Bore has a mind of her own.
0:01:48 > 0:01:53I'm sure there's something moving up there. The river's lost its shine.
0:01:53 > 0:01:56All I can hear my own heart pumping like mad.
0:01:56 > 0:01:58PEOPLE WHOOP
0:01:58 > 0:02:03Breaking waves! There's breaking waves on the far side. Here it is, this is fantastic!
0:02:03 > 0:02:07Must be about 200 metres away, and I can see the breaking wave already.
0:02:07 > 0:02:09CHILDREN CALL: Look! What's that?
0:02:09 > 0:02:12This placid river is suddenly being ripped up
0:02:12 > 0:02:16and all the laws of nature have been thrown into reverse, broken,
0:02:16 > 0:02:20because there's a great standing wave just MOVING...unstoppably
0:02:20 > 0:02:23the wrong way. It's coming upstream.
0:02:23 > 0:02:26- And it looks like something living! - LOUD RUSH
0:02:30 > 0:02:31And it's breaking on this side, too,
0:02:31 > 0:02:35so we've got waves on both sides, this huge wall of surf that's
0:02:35 > 0:02:39breaking out right close to me here and going out 15 metres, fantastic!
0:02:39 > 0:02:41Absolutely incredible.
0:02:41 > 0:02:45There are more waves following it.
0:02:45 > 0:02:47My god, that huge wave!
0:02:48 > 0:02:50Fantastic!
0:02:50 > 0:02:54Run quick, because the river goes through some bends further upstream,
0:02:54 > 0:02:58if I'm quick I'll be able to cut the Bore off and meet it further up.
0:03:06 > 0:03:09The Bore first appears some five miles inland from the sea
0:03:09 > 0:03:12as the Severn suddenly bottlenecks.
0:03:12 > 0:03:14Then it takes the best part of an hour
0:03:14 > 0:03:17to follow the river's twists and turns to Minsterworth, where I am.
0:03:17 > 0:03:22And in another five minutes or so, it should reach Minsterworth church.
0:03:24 > 0:03:26CYCLE BRAKES SQUEAK
0:03:26 > 0:03:29Made it. What's the time?
0:03:29 > 0:03:31I reckon I've got about a minute at most
0:03:31 > 0:03:35until the Bore comes creaming round that corner down there.
0:03:35 > 0:03:37SOUND OF RUSHING WATER
0:03:37 > 0:03:39Now I've seen it once,
0:03:39 > 0:03:45I'm beginning to understand why thousands of years ago, the people who lived along this riverbank
0:03:45 > 0:03:48looked at this wave with awe and a lot of incomprehension.
0:03:51 > 0:03:57In fact, in ancient Welsh, this was just known as "the roaring wave".
0:03:57 > 0:03:59Absolutely awesome.
0:04:04 > 0:04:07From Minsterworth, the Bore continues relentlessly,
0:04:07 > 0:04:11ripping at the river banks all the way to Gloucester.
0:04:11 > 0:04:13But make no mistake about it,
0:04:13 > 0:04:18what we're looking at isn't caused by the tide - the Bore IS the tide.
0:04:18 > 0:04:21It's the raging sea 20 miles inland.
0:04:21 > 0:04:23The entire river has been forced backwards,
0:04:23 > 0:04:26and those waves taste of salt!
0:04:27 > 0:04:32But if I'm to find out exactly what causes the Severn Bore,
0:04:32 > 0:04:37at least part of the answer must lie 240,000 miles up there in the night sky,
0:04:37 > 0:04:41because what I do know is that it's not Britannia that rules the waves,
0:04:41 > 0:04:43it's the Moon.
0:04:44 > 0:04:47Just like two kids spinning in a playground,
0:04:47 > 0:04:51the Earth and Moon are in a constant pirouette around each other
0:04:51 > 0:04:53and also around the Sun.
0:04:53 > 0:04:57As it rotates around our planet, the Moon exerts a gravitational pull
0:04:57 > 0:05:00on the greatest single mobile mass on Earth, the sea,
0:05:00 > 0:05:06which is physically moved backwards and forwards to give us high and low tides.
0:05:06 > 0:05:12Although much further away from Earth, the Sun also exerts its own massive gravitational pull.
0:05:12 > 0:05:17So when, on occasions, the Sun, Moon and Earth align in a straight line,
0:05:17 > 0:05:20the pull of the Sun and Moon together almost doubles the effect.
0:05:20 > 0:05:26And it's no surprise that what we get on Earth is exceptionally high or "spring" tides -
0:05:26 > 0:05:30exactly when we get the biggest bores on the River Severn.
0:05:30 > 0:05:32So the Sun and Moon cause the tides,
0:05:32 > 0:05:39but the Severn Estuary has the second highest tides not just in the UK, but in the entire world.
0:05:39 > 0:05:43Second only to The Bay of Fundy in Canada. Why?
0:05:43 > 0:05:47Well, the morning after the night before, the tide has gone into full reverse.
0:05:47 > 0:05:53In search of answers, I've met up with oceanographer Chris Wooldridge.
0:05:53 > 0:05:56Tide's now pouring back out to the Atlantic
0:05:56 > 0:05:59and the buoy's being tilted over by the force of the water, isn't it?
0:05:59 > 0:06:02It's beginning to go like a train, you know.
0:06:02 > 0:06:09How does the shape of the Bristol Channel, the Severn Estuary, convert into this tidal wave further inland?
0:06:09 > 0:06:11Because down here it looks fairly placid.
0:06:11 > 0:06:15Well, the very shape itself - an ever-narrowing funnel -
0:06:15 > 0:06:18plus the length of the basin, the length of the estuary,
0:06:18 > 0:06:20and the gradient and shape of the seabed.
0:06:20 > 0:06:26These are the unique factors that combine to trigger the Severn Bore.
0:06:26 > 0:06:29This wall of water brought in from the Atlantic,
0:06:29 > 0:06:31it's got to go somewhere.
0:06:31 > 0:06:36It's been dragged across the ocean by the pull of the Moon, and the landmass wants to stop it.
0:06:36 > 0:06:42But that tidal wave is going to run on, forced into an ever-narrowing funnel and forced up at speed.
0:06:42 > 0:06:47So you've got the whole Atlantic Ocean squeezing up this funnel and then rushing upstream.
0:06:53 > 0:06:55Wow!
0:07:01 > 0:07:04Uh-oh! WATER ROARS
0:07:04 > 0:07:09Unlike the Northern Lights or a double rainbow, the Bore adheres to a strict timetable
0:07:09 > 0:07:15and it's no surprise that crowds flock to see it, nor that many feel the urge to ride it. To tame it.
0:07:15 > 0:07:19The beauty is that with two tides a day, there's a twice-daily rodeo.
0:07:24 > 0:07:27What is amazing is that for hundreds, if not thousands of years,
0:07:27 > 0:07:32people have apparently struggled to LIVE with these tides rather than running a mile.
0:07:35 > 0:07:39Professor Martin Bell has spent 20 years examining evidence of this struggle
0:07:39 > 0:07:45preserved deep in the thick tidal mud of the Gwent Levels, between the Severn Bridge and Cardiff.
0:07:45 > 0:07:51Martin has promised to show me something incredible preserved in this mud. My first impressions?
0:07:51 > 0:07:57Well, the mud-flats of the Gwent Levels don't have quite the instant appeal of the Valley of Kings
0:07:57 > 0:07:59or even Salisbury Plain!
0:07:59 > 0:08:06This grey, claylike material is laid down in layers, like layers of... cake icing, eh, except more skiddy?
0:08:06 > 0:08:13Yes, and these sediments, banded sediments, incredibly, preserve human footprints.
0:08:13 > 0:08:15- So you can see... - Is that what these are?!
0:08:15 > 0:08:20- Yes, these are actually Mesolithic human footprints. - That is astonishing!
0:08:20 > 0:08:25Made in the soft mud, but now semi-consolidated as the whole things become compressed.
0:08:25 > 0:08:27As you see, they're quite small.
0:08:27 > 0:08:32They are obviously mostly children of seven, eight, nine, probably.
0:08:32 > 0:08:35So roughly when do these footprints date from?
0:08:35 > 0:08:417,000 years ago, at a time of very rapid sea-level rise, when the whole estuary was really inundated.
0:08:41 > 0:08:44Previous to that, there'd been a bay
0:08:44 > 0:08:50stretching between Pembrokeshire and Devon, but suddenly this funnel-shaped estuary opened up.
0:08:50 > 0:08:55It's at that time that the huge tidal range, 14.8 metres, would've developed.
0:08:55 > 0:08:58- That was the beginning of the Severn Bore?- Exactly.
0:08:58 > 0:09:03- These children could've seen the first tides that caused the Severn Bore?- Yes.
0:09:03 > 0:09:06SOUNDTRACK: CHILDREN LAUGH...
0:09:06 > 0:09:08..WATERFOWL CALL
0:09:20 > 0:09:22Subtitles by Red Bee Media Ltd - 2006
0:09:22 > 0:09:24Email subtitling@bbc.co.uk