Why can submarines pass Gibraltar with engines off?

Because two currents run through it in opposite directions at different depths. The Mediterranean evaporates faster than its rivers replace, so cooler Atlantic water flows in at the surface while denser, saltier Mediterranean water flows out beneath it - a permanent two-layer conveyor in a channel thirteen kilometres wide.

A submarine that sinks into the lower layer is carried west into the Atlantic, and one riding the surface layer is carried east. German U-boats used exactly this in the Second World War to slip past Allied listening stations and patrols, drifting silently rather than running engines that could be heard. Going the other way was much harder and several boats were lost trying.

The same exchange is why the Mediterranean is as salty as it is, and why it is nearly tideless - the flow through the gap is small relative to the basin. It also very nearly closed for good: about six million years ago it did close, and the sea evaporated into a deep basin of salt flats before the Atlantic broke back through in a flood that may have refilled it in a matter of months.