Which sea separates Europe from Africa?

The Mediterranean, and it has dried up completely at least once. Around six million years ago the connection at Gibraltar closed, and the sea evaporated over a few thousand years into a deep hot basin of salt flats two or three kilometres below the surrounding land - the Messinian salinity crisis, discovered by drilling into a kilometre-thick layer of salt under the sea floor.

It refilled catastrophically. When the Atlantic breached Gibraltar again, the water is thought to have poured in at a rate that could have filled the basin in a matter of months to a couple of years, in what would have been the largest flood in the planet's history by a wide margin.

The modern sea is still nearly closed, which is why it is so salty and almost tideless - the exchange through the fourteen-kilometre gap at Gibraltar is small relative to how much evaporates. That near-enclosure is why it became the highway of the classical world: manageable distances, few tides, and predictable summer weather. It is also why it is so vulnerable now, with warming, invasive species arriving through the Suez Canal, and some of the highest concentrations of floating plastic anywhere.