Which strait separates mainland Europe from the United Kingdom at its narrowest point?

The Strait of Dover is the pinch point where the English Channel meets the North Sea and Britain comes closest to France, close enough that the white chalk cliffs on either side are visible from the opposite coast on a clear day. Those cliffs are the clue to the geology: the same band of chalk runs beneath the water, and Britain was joined to the continent by dry land during the ice ages when sea levels were far lower, with the strait opening after catastrophic flooding breached a ridge and drained a huge lake trapped behind it. That continuity of soft, stable chalk is what made the Channel Tunnel practical, since boring machines could follow a single geological layer across. The strait is also one of the busiest shipping lanes anywhere, controlled by a separation scheme that keeps opposing traffic apart, and it is the classic course for long-distance swimmers.

The Channel Tunnel carries trains through the chalk beneath the strait rather than resting on the seabed.