Timubo Cave

A flight of steps drops through a tight passage — low enough in places that taller visitors go down sideways — into a chamber holding a pool of very clear, very cold fresh water, chest-deep and lit by a string of bulbs.

The swim is the whole point and takes as long as you want it to. The passage down is the part to think about: it is confined, damp and slippery, and there is one way in and out.