The night the North Sea came in

A deep depression drove the sea south down the North Sea and it arrived at the coast on a spring tide, two days after the full moon.

The water reached more than five metres above mean sea level; 307 people died in England and over 1,800 in the Netherlands, most of them in their beds because there was no way to warn them. Both countries built their modern sea defences in answer to it, and the Thames Barrier exists because of this one night.