It's 10 o'clock on a Sunday evening, and I'm standing on the banks of the River Severn in the Gloucestershire village of Newnham on Severn. The church bells mark the hour, and a ripple of excitement runs through the crowd next to me.
In just a few minutes, the Severn's lazy meander to the Bristol Channel will be interrupted by a huge tidal surge, one of Britain's most spectacular natural phenomena. This is the Severn Bore, a wave described by Thomas Harrel in 1824 as one that "rolls in with a head... foaming and roaring as though it were enraged by the opposition which it encounters".
The prospect of a raging Severn Bore has lured the South African big-wave surfer Duncan Scott to Gloucestershire. Scott is based in the UK, but is fresh from a trip to his homeland, where he has just surfed Dungeons, a break on the Western Cape Peninsula.
"Surfing Dungeons is a real adrenaline rush," says Scott, 28, who survived - and enjoyed - a session there in surf of 25-30ft, on a day which also saw the Californian surfer Greg Long ride one rogue wave estimated at 70ft. "But I love surfing the Bore," he adds, before adapting the famous surfers' cliché: "You should try it. Only a Bore surfer knows the feeling."
I watch Scott slide down the muddy bank and into the darkened water. He drifts across the river with two other surfers. I cast my eyes upriver. Some light is given off by The White Hart Inn half-a-mile away, but the moon casts a diffuse, lacklustre glow. How on earth will the surfers be able to see where they are going?
These thoughts are disturbed by a strange hissing sound. The crowd surges forward. Sure enough, the Bore is on its way, a mass of white water spanning the river, some three foot in height. Scott, visible thanks to a yellow safety jacket, leaps nimbly to his feet, while his companions opt to "prone" it, lying flat on their boards, chests raised.
As the Bore passes I see Scott leading the journey upriver, the Severn's flat, tranquil waters now dominated by the inexorable incoming tide. It is a sight at once surreal and inspiring, and I cannot help but think: shall I give it a go, too?
To surf the Severn Bore is to enter a unique club pioneered by a Commando officer, Colonel Jack Church-ill, in 1955. "Mad" Jack was known for his eccentric exploits, including the deployment of a longbow in a Second World War battle, and earned his unlikely place in surfing history by entering the Severn with a home-made 16ft surfboard at Stonebench on 21 July 1955.
His brief ride was not replicated until 1962, when, against police advice, visiting Australian lifeguards surfed the Bore for a mile. Since then, it been surfed by a dedicated crew of locals and visitors.
The warnings, though, are made for good reason. At 50ft, the River Severn's tidal range is the second highest in the world, meaning an incredible amount of water is propelled up an increasingly narrow channel. Bores occur year-round with spring tides, but the biggest are around the vernal equinox (February, March and April) and autumnal equinox (August, September and October). Then, if there is also a low- pressure system and strong south to south-westerly winds, the funnel-like shape of the Severn Estuary can create a wave as high as six feet. To add to the difficulty, another imponderable is the amount of debris in the water: tree branches, oil cans, tyres, window frames and fridges are just some of the obstacles that a surfer might encounter.
A surfer who loses the wave is at the mercy of the river, whose current could by then be travelling at up to 12mph, not to mention other hazards. These include tangles of branches, high cliffs and whirlpools.
Indeed, Trevor "The Whirlpool" Stephens, 39, a North Devonian who regularly surfs the Bore - earned his nickname precisely because of the latter.
"I got knocked off one night by James, a friend who surfs here a lot," he recounts. "There was a ferocious current and I was heading straight to the salmon-catchers at the White Hart. I missed them by about three feet but then got caught in a whirlpool further along, underneath some cliffs."
His friend, James Golding, picks up the story: "The only way out was to climb the cliffs. So he climbed a 50ft cliff, knowing that if he fell he'd be right back in the whirlpool again."
Both Stephens and Golding insist that the Bore has to be respected. As Stephens says: "When it goes wrong here, it goes badly wrong." There are local heroes: men such as Steve King and Dave Lawson, who have vied with each other for the world record for the longest wave-ride (honours currently lie with King, with a 7.6-mile ride last April), but there is no surf-dude arrogance, and none of the aggression that can go with surfing a popular break.
"Surfing here is unique," says Scott. "Where else can you cruise on a board for miles, riding past houses, trees and beautiful fields?"
It's Monday morning, and I have spent three days watching Scott, Golding, Stephens and other Bore regulars having the time of their lives. Scott may have been hoping for a Bore of maximum size, but the absence of wind and high pressure made for twice-daily tidal surges of between one and three feet - perfect for a Bore novice. I am tempted, but the longest board I own is 7ft 6in, and Scott cautions that anything less than nine feet would not do the trick.
But it just so happens that he has a spare strapped to his roof-rack, and so here we are the next morning at Newnham, waiting for the Bore.Near us in the water is Steve Hislop, a local sculptor whose life revolves around surfing the Bore, as well as Stephens and other regulars. At last, the Bore heaves into view. We catch it; Scott is on his feet straight away, but I fall as soon I am up. Cue frantic paddling to make the exit point, where I am helped out by Stephens. I caught the wave, and felt its power, but I then I blew it.
Or had I? Scott announces that we will jump in the cars, chase the Bore and find another take-off point. And there, as he rides an 11ft Tiki board in tandem with his girlfriend, Hannah Pearn, I catch an exquisite two-foot wave which peels, surges and reforms. I ride it for perhaps three quarters of a mile, and without a shadow of a doubt, it is the most memorable wave of my life.
For more information: severn-bore.co.uk