The blue cave of Bisevo is not terribly easy to visit.
First, you must get yourself to the Croatian island of Vis, by ferry from the city of Split. Once there, travel by bus or motorbike to the other side of the island, the tiny fishing town of Komiza. There, hire a boat or tour to take you to the island of Bisevo, about an hour boat ride from Komiza. Then you will approach the cave via a small rubber raft, which is just small enough to enter the tiny cave entrance.
The Blue Cave (Modra špilja) is located at the Balun Cove on the eastern side of the island. Though Croatian fisherman have known of the blue cave since ancient times, it wasn't until 1884 that a (relatively) easy entrance was blasted out with dynamite. Before this small hole in the side of the rock was opened, the only entrance into the cave was to dive underneath the rock wall in just the right spot.
The cave is at its most beautiful between 11 am and 12 pm on a sunny day. The bright blue glowing effect is created when the sun's rays enter through the water and reflect off the limestone floor of the cave. The bright sun lights up the water, and the glowing ocean illuminates the cave walls a brilliant blue, creating a ethereal glowing blue grotto.
Though not an easy trek, it is a rewarding one - the blue cave is a truly enchanting natural wonder.
via Atlas Obscura http://www.atlasobscura.com/places/blue-cave-bisevo