Blue Cave is a water-logged sea cave on the east side of the island of Biševo, in central Dalmatian archipelago of Croatian Adriatic. It is one of the best known natural beauty spots on the Adriatic and a popular show cave due to the the glowing blue light that appears at certain times of day.
The Blue Cave was at first time described and painted by Baron Eugen von Ransonet. It was originally accessible only by diving, but based on his suggestion, an artificial entrance large enough for small boats was built in 1884.
With natural entrance to the cave, located on its southern side, it resembles a vault on the ceiling of the cave. It is through this submarine-like opening on the ceiling of the cave that leaves sunlight in and creates an iridescent blue glowing effect all around the cave. A stone bar is connecting with two walls of the cave and is clearly visible just below the waterline.
Depending on the season, the ideal moment to visit the cave is between 11 a.m. and 12 noon. At this time of day the sunlight reflects through the water coming from the white floor of the cave and bathes the grotto in aquamarine light, while objects in the water appear to be silver.
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 meters deep and up to 15 meters high, while the entrance measures 1.5 meters high and 2.5 meters wide.
The cave attracts more than 10,000 tourist visits every year, but the tourist boats often include also a visit to another cave on the island, the Green Cave (Zelena špilja). Green Cave is similar to the Blue Cave but it is bigger in size and its emerald-green tone have a similar effect.