The Blue Cave or Blue Grotto or Modra špilja is a splendorous and unique geomorphological natural wonder on the Croatian Adriatic located in a bay of Balun on the island of Biševo, about 9 km (5 miles) from Komiža, Vis, and over 69 km (111 miles) from Split.

A hypnotizing silver-blue light illuminates the space inside the cave when the sunlight hits the surface of the ocean through a crack or vault (natural opening) on its arched roof and reflects off the seabed. The intensity of the blue color varies with season and time of the day. It’s best visited on a bright sunny day between 11 am and 12 pm as that’s when the sun is just at the right angle intensifying the entire experience.

How’s the Blue Cave formed?
Over a period of time, the ocean waves eroded the limestone rock to form the Blue Cave. The cave is pretty tiny (24 m long, about 10 to 12 m deep, and around 15 m high) with a narrow entrance (1.5 m high and 2.5 m wide).
Accessible only by diving through the natural opening (vault on the ceiling of the cave) until 1884 before it was discovered by Baron Eugen von Ransonet, the cave can now be visited on a small boat through a narrow artificial opening blasted out with dynamite.

About Biševo Island
Biševo Island, right in the center of the Dalmatian archipelago is only a few square miles in size. It’s composed of limestone rock and is the remotest and least populated inhabited Croatian Island with just about 15 residents.
There are no accommodation options on the island, hence the travelers only disembark here for a short while. Porat (Biševska Luka) and Salbunara bays on the island of Biševo have gorgeous sandy beaches if you want to swim and get tanned. But make sure you carry your beachwear.
The other attractions on the island are the Benedictine monastery ruins and the 11th-century well-preserved church of Saint Sylvester or Sveti Silvestar.
Medvidina spilja or Monk Seal Cave is another cave on Biševo Island in the Tresjavac bay.