The Blue Cave on Biševo Island is a water-logged sea cave located in a small bay called Balun (ball in local dialect), about 4.5 nautical miles (8.3 km) from Komiža town on Vis Island, in the Croatian Adriatic. The cave 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 mid-day.
First described and painted by Baron Eugen von Ransonet, the cave was originally accessible only by diving as it had one natural entrance below the sea level. Based on his suggestion, an artificial entrance large enough for small boats was built in 1884.
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.