Blue Cave, Biševo
Concierge.com's insider take:
Just across the water from Vis, the island of Biševo is famous for its waterlogged rock cavern, locally called the Modra Špilja, that can only be reached by boat. For an hour or so a day, usually beginning around 11 a.m., the grotto seems to glow from underneath with an incandescent blue light. Charter boats leave every morning from Komiža, on Vis, though once there, you'll have to swim or hop on a rowboat to get inside the cave. And while there's no question that most visitors feel an otherworldly connection to the place, on summer days it can get distractingly crowded.