The Blue Grotto or Blue Cave (Croatian: Modra špilja), is a water-logged sea cave located in a small bay called Balun (Ball in the local dialect), on the east side of the island of Biševo and about 4.5 nautical miles (8.3 km) from Komiža. 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. 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 natural entrance to the cave, located on its southern side, is said to resemble a vault on the ceiling of a grotto. It is through this submarine-like opening on the ceiling of the cave that sunlight gets in and creates an iridescent blue glowing effect all around the cave. As well, a stone bar, connecting two walls of the cave, is clearly visible just below the waterline, both in above-water photographs and in underwater photos.