Britain might be breaking away politically from the European Union right now, but researchers have found proof that a “geological Brexit” started more than 4,50,000 years ago. A paper published in the journal Nature Communications suggests that what is now the UK was once connected to Europe through a thin strip of land that was later flooded by a large lake.

“This was really one of the defining events for north west Europe – and certainly the defining event in Britain’s history,” said Professor Sanjeev Gupta of Imperial College London, who led the study. “This chance geological event, if it hadn’t happened, would have meant Britain was always connected to the continent.”

The strip of land connected Dover in the South of England to Calais in northern France, and had a large glacial lake right next to it, claims the paper. The land, according to the scientists, was first submerged when the lake overflowed and then completely destroyed by a second catastrophe around 1,50,000 years ago. The researchers said that the large underwater holes found at the bottom of the English Channel were formed when the lake flooded.

However, the geologists are not entirely sure why the lake overflowed. “Perhaps part of the ice sheet broke off, collapsing into the lake, causing a surge that carved a path for the water to cascade off the chalk ridge,” said co-author Jenny Collier from Imperial College London. “In terms of the catastrophic failure of the ridge, maybe an earth tremor, which is still characteristic of this region today, further weakened the ridge.”

British Prime Minster Theresa May on March 29 triggered the Article 50 of the Lisbon Treaty to start the formal process of UK’s departure from the EU. The UK had voted to leave the EU in a referendum on June 24, 2016,