“It is probably one of the largest floods ever identified,” said Phillip Gibbard, a geographer at the University of Cambridge who wasn’t involved in the study. At its peak, the flood would have discharged water at a rate of about 264 million gallons (a million cubic meters) a second, gushing at speeds of up to 62 miles (100 kilometers) an hour, the researchers say. This is roughly equivalent to ten times the combined flow rate of all the rivers in the world.

In addition to making Britain an island, the authors add, the huge flood had wide-ranging environmental consequences. For example, the gigantic pulse of freshwater entering the Atlantic Ocean likely caused a period of climate cooling in the Northern Hemisphere, Gibbard said. “The introduction of ice and freshwater into an ocean drives climate oscillations and causes marked cooling events,” he explained.

The flood also marooned many animals and plants, so those species gradually evolved into different forms than their mainland cousins.