About half a million years ago, in the midst of an Ice Age, a land bridge connected Dover in the South of England to Calais in northern France.
Immediately to the north of it, was a huge glacial lake, which had formed at the edge of the ice sheet that covered much of Europe.
When it started to overflow, vast amounts of water crashed over the land bridge, forming a series of very large waterfalls.
The lake overflowed 450,000 years ago, damaging the land link. Then a later flood fully opened the Dover Strait.
The scars of these events have recently been found on the seabed of the English Channel. The study is published in the journal Nature Communications by S. Gupta et al. on 04 April 2017.
Decades ago, engineers who were surveying the seabed for the Channel Tunnel, discovered a series of mysterious large underwater holes (fosse) between Dover and Calais, caused by the lake overspill (black regions on the map above) The fosse are now in-filled with sediment and show up as a line of isolated depressions 100m-deep carved into the bedrock and hundreds of metres to several kilometres in diameter.
450,000 years ago, the glacial lake water plunged over the rock ridge as a series of waterfalls from Dover to Calais, which then eroded and carved out these depressions.
A second catastrophic flood took place about 150,000 years ago forming a huge valley about 10km wide with a lot of features suggesting flood erosion. Perhaps an ice sheet broke off, collapsing into the lake, causing a surge that carved a path for the water to cascade off the chalk ridge. As the channel floor slowly eroded by these torrential floods, seawater from the Atlantic Ocean rushed into the resulting channel, isolating the British Isles from the mainland.