Britain became separated from mainland Europe after a catastrophic flood some time before 200,000 years ago, a sonar study of the English Channel confirms.
The images reveal deep scars on the Channel bed that must have been cut by a sudden, massive discharge of water.
Scientists tell the journal Nature that the torrent probably came from a giant lake in what is now the North Sea.
Some event - perhaps an earthquake - caused the lake's rim to breach at the Dover Strait, they believe.
Dr Sanjeev Gupta, from Imperial College London, and colleagues say the discharge would have been one of the most significant megafloods in recent Earth history, and provides an explanation for Britain's island status.
"This event, or series of events, that caused [the breach] changed the course of Britain's history," Dr Gupta told BBC News.
"If this hadn't happened, Britain would always have been a peninsula of Europe. There would have been no need for a Channel Tunnel and you could always have walked across from France into Britain, as early humans did prior to this event."
The idea of a great flood stems from scientists' understanding of northern Europe's ice age past.
It is believed that hundreds of thousands of years ago, when ice sheets had pushed down from Scotland and Scandinavia, there existed a narrow isthmus linking Britain to continental Europe.
This gently upfolding chalk ridge was perhaps some 30m higher than the current sea level in the English Channel.
Palaeo-researchers think it bounded a large lake to the northeast that was filled by glacial meltwaters fed by ancient versions of the rivers Thames and Rhine.
Then - and they are not sure of the precise date - something happened to break the isthmus known as the Weald-Artois ridge.
"Possibly this was just the build-up of water behind. Possibly something triggered it; it's well known today that there are small earthquakes in the Kent area," explained Imperial's Dr Jenny Collier.
Either way, once the ridge was broken, the discharge would have been spectacular.
The Imperial College and UK Hydrographic Office study used high-resolution sonar waves to map the submerged world in the Channel basin.
The images detail deep grooves and streamlined features, the hallmarks of landforms that have been gouged by large bodies of fast-moving water.
At its peak, it is believed that the megaflood could have lasted several months, discharging an estimated one million cubic metres of water per second. And from the way some features have been cut, it is likely there were at least two distinct phases to the flooding.
"I was frankly astonished," said Dr Collier. "I've worked in many exotic places around the world, including mid-ocean ridges where you see very spectacular features; and it was an enormous surprise to me that we should find something with a worldwide-scale implication offshore of the Isle of Wight. It was completely unexpected."
The researchers tell Nature that the ridge breach and the subsequent flooding would have helped reorganise river drainage in northwest Europe, re-routing both the Thames and the Rhine.
The megaflood theory has been around for some 30 years; but the sonar images represent the clearest narrative yet for the story.
Previous studies of prehistoric animal remains from the past half-million years have already revealed the crucial role the English Channel has played in shaping the course of Britain's natural history.
The Channel has acted as a filter through time, letting some animals (including humans) in from mainland Europe but not others.
And even when water was locked up in giant ice sheets and sea levels plummeted, the Rhine and the Thames rivers would have dumped meltwater into a major river system that flowed along the Channel's floor.
Scientists can see all of this influence written in the type and mix of British fossils they find at key periods in history.
Professor Chris Stringer is director of the Ancient Human Occupation of Britain (Ahob) project, which has sought to fill out the details of the British Isles' prehistory.
"The timing and method of formation of the Channel has been a long-running argument - after all, it really makes Britain what it is today, geographically," he commented.
"The evidence presented in this paper is spectacular. It certainly explains and reinforces the picture the Ahob project has been putting together of the increasing isolation of Britain from Europe after 400,000 years ago."