Today you can take a train, in a tunnel beneath the English Channel called the Chunnel, from London to Paris in 2 hours.

But half a million years ago, the English Channel didn’t exist: Britain was attached to Europe.

At that time, Earth alternated between glacial and interglacial periods, as it still does today.

During glacial periods, some of Earth’s water is held in vast glaciers, on top of continents rather than in oceans, which causes ocean levels to drop hundreds of feet.

Toward the end of one of these glacials, as the ice sheets started to melt, an enormous lake formed in the North Sea—dammed in the north by a massive wall of ice and in the south by a rock land bridge.

Eventually water levels rose so high that the lake began pouring over the British land bridge in gargantuan waterfalls.

Engineers planning the path of the Chunnel discovered this. Using seismic studies, they found huge, deep pits in the seafloor filled with rubble and sediment, and designed the Chunnel to avoid them.

Eventually scientists collected enough data to recognize the pits as plunge pools that those giant waterfalls would have made from millions of tons of water, pounding down on the seafloor over centuries.

The waterfalls began to erode the land bridge, but it held—until, a couple of glacial and interglacial cycles later, another flood finished the job.

The channel formed—causing a natural Brexit thousands of years ago.