Blue Cave: Not only is the Blue Cave one of the unique natural phenomena in the world, but this bright and beryl attraction on Bisevo island is destined to be the highlight of your Croatian holiday. The magnificent work of Mother Nature, this cerulean oasis is so unique because it is formed due to seawater eroding limestone, a rock Biševo island is wholly composed of. Thanks to the sunlight’s reflection off of the white seabed, which creeps through the opening of a vault at the top of the cave, a mesmerizing blue hue fills its capacity.