The narrow seaway that separates Britain from mainland Europe has shaped history and culture. However, how the English Channel formed has long been a mystery. A massive flood was one popular idea, but evidence to support it was lacking. Now, an analysis of data collected over a 24-year period indicates that at least two catastrophic floods breached an isthmus at the Dover Strait, allowing an ice-dammed lake to pour into what is now the English Channel.

Sanjeev Gupta, of Imperial College London, never set out to learn how Britain became an island. Then, in 2003, the field geologist came across a book describing various hypotheses of how Britain formed — but scant evidence to support a prevailing idea. Realizing technology could now offer a solution, Gupta discussed the problem with his colleagues, geophysicist Jenny Collier, an expert on ocean-floor mapping, and postdoc Andy Palmer-Felgate. But there was a problem: collecting the data would mean taking a small boat back and forth across one of the world's busiest shipping lanes.

During a separate collaboration, Graeme Potter from the UK Hydrographic Office in Taunton provided a solution. For 24 years, his office had been surveying large swathes of the English Channel so that ships could sail more safely. He offered the team these data and a large map — the first image of the English Channel floor. “We could see this huge valley,” recalls Gupta. “There was something peculiar about it.”

A few months later, the Imperial College team began exploring the data using three-dimensional mapping tools. The researchers determined that the strange valley had a bedrock floor with streamlined islands scattered down its axis. Islands such as these typically form when so much water enters a river bed that the channel splits in two, leaving a raised strip of land in between (see page 342). Gupta saw similarities between these landforms and those of the Channeled Scabland in the US state of Washington. These were attributed to a megaflood that occurred 15,000 years ago.

By contrast, the British megafloods probably occurred between 450,000 and 180,000 years ago. A lake, impounded where the North Sea is now, breached a rock dam at the Dover Strait and floodwaters flowed southwest. The flow, which may have lasted for months and reached peak discharges of a million cubic metres per second, carved a valley, 50 metres deep, into the rock. It would have been a tsunami-like wall of water, says Gupta.

The discovery adds weight to an old idea. The possibility of the existence of a lake where the North Sea is now was mentioned as far back as 1878. And, says Gupta, a 1985 paper proposed that a huge flood had established the English Channel, although the authors provided little evidence for their outlandish hypothesis, and the article sunk into obscurity.

Until reports such as these piqued Gupta's curiosity, most of his research had focused on interpreting desert rock formations. But the discovery of catastrophic floods in his own backyard has taken his work beyond the remotest corners of the Earth — searching for evidence of similar flooding on Mars.