Gljúfrafoss, Iceland

There’s nothing more mysterious than a waterfall cascading down into a cave, which is exactly what you get with this place. Getting to the waterfall is not for the claustrophobic: you’ll have to slide through a narrow crack in the rock to get into the cave where you can see the waterfall. But once you’re inside, you’re sure to be impressed by the look of the falls, which seem to come out of nowhere. There’s also a viewing platform about halfway up the falls for a different sort of view—just make sure you wear waterproof shoes, as the spray tends to get your feet a bit wet.