Down the muddy bank they slipped and slid, into the murky river at Magdalen.
Seven intrepid wild swimmers had come to tame the Wiggenhall Wave.
While tiny tidal waves happen each time the tide turns in the Great Ouse, you'd miss most of them if you blinked.
But when full moon shines over the Fens and spring tides surge up The Wash, the flood creates rollers 2ft or more high as it piles into the Lynn Channel and up the New Cut. They call it the Wiggenhall Wave in those parts.
As the mists cleared around 6.20am, the bore rounded the bend by St Germans church, and came rushing towards the swimmers, who screamed and cheered as it picked them up and carried them away up the river.
'We thought it was worth getting up to see it,' said villager George Garner, 63, who was waiting on the bridge with his wife, Linda, to see the swimmers.
Asked if he'd considered joining them, he admitted he thought those taking part were 'definitely not bolted together right'.
By the time the bore passed Stowbridge, where the swimmers hoped to exit the river, it had divided into a series of waves.
Renewable energy entrepreneur Kevin Holland, who lives on the banks of the river, predicts and publishes tidal wave times.
'I predicted this about six months ago and it was spot on,' he said. 'I've canoe-ed up it. You're being carried along by the tide, you can't see the sky, you just see these green banks on either side, it's a surreal experience.'
Mr Holland, who believes the phenomenon could become a tourist attraction in its own right, added: 'I'm, going to make the Wiggenhall Wave into the east of England's Severn Bore.'
A common seal bobbed into sight before the swimmers, diving around Stowbridge before making a beeline for Denver.
Then the swimmers appeared – after their two mile swim – and waded through the margins to their waiting cars. Retired teacher Dawn Maycock, from Norwich, said: 'It was excellent.'
Zara Bullen, from the city's Golden Triangle, who swam the Channel in 2013, said: 'This was just a fun swim to keep me out of mischief.'
Asked why they chose to take to the unpredictable tidal river rather than more hospitable waters, Bryn Dymott, from St Neots, said: 'It's the freedom. The tricky bit was getting into the water.'
Have you ever swum a tidal bore? Email chris.bishop@archant.co.uk.
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