Which strait separates Europe from Africa at its narrowest?

Gibraltar, about 13 kilometres across at the tightest point - close enough that the two continents are visible from each other on any clear day, and close enough that it has been crossed by everything from Neanderthals to migrating storks, which cross there because they need thermals and cannot glide far over open water.

It is also a one-way valve. The Mediterranean evaporates faster than its rivers can supply, so cooler Atlantic water flows in at the surface while denser, saltier Mediterranean water flows out beneath - two currents in opposite directions in the same channel. Submarines have used the lower one to pass through without engines, which is exactly what German U-boats did in the Second World War.

The strait closed once, about six million years ago, and the Mediterranean dried into a deep hot basin of salt flats - the Messinian salinity crisis, discovered by drilling into a kilometre-thick salt layer under the sea floor. When the Atlantic broke through again, the refilling flood may have been the largest in the planet's history, possibly filling the basin in a matter of months to a couple of years.