The Blue Grotto or Blue Cave, is a water-logged sea cave located in a small bay called Balun, on the east side of the island of Bisevo and about 4.5 nautical miles (8.3 km) from Komiza, in the Croatian Adriatic.
It is situated in the central Dalmatian archipelago, 5 km south-west of the Island of Vis. The grotto is one of the best known natural beauty spots on the Adriatic and a popular show cave because of the glowing blue light that appears at certain times of day.
The cave was formed by the wave action of the sea, as sea water eroded the limestone rock of which the whole island of Biševo is composed. The cave itself is 24 meters long, 10–12 metres deep and up to 15 metres high, while the entrance to it measures 1.5 metres high and 2.5 metres wide.
The cave receives more than 10,000 tourist visits every year, and tourist boats often include a visit to another similar cave on the island, the Zelena špilja (English: The Green Grotto), which is bigger in size and appears to be of an emerald-green tone due to a similar effect.