In 1884 the Viennese painter Baron Eugen Ransonnet introduced the Blue Cave on the island of Biševo to the world. His discovery marked the beginning of tourism in Dalmatia, and the Blue Cave has since become a must-see tourist attraction of the Adriatic. The Blue Cave is protected as a geomorphological monument of nature since 1951. A special natural light phenomenon in the cave leaves no one indifferent. Ransonnet wrote that this rich light source gave the rocks, the ores and other submerged objects a silvery-bluish hue similar to the one in the Capri Blue Grotto, directing light from all sides onto the rocky reef, making it look like a castle of a sea deity. The geological survey of the cave in 2019 shed further light on the phenomenon. The bottom of the entrance area of the Blue Cave, as well as the bottom of the big cavern is sandy and light-yellow in colour. In the morning, when the sun shines from the east onto the sandy bottom through the submerged entrance to the Blue Cave, a large quantity of light reflects from the sand (which is lighter than the surrounding rocks of the Blue Cave that have a biogenic overgrowth) and illuminates the interior of the cave. In addition to the submerged entrance, the cave has two more entrances, by sea and by land. The semi-submerged cave is inhabited by numerous organisms living in semi-darkness. During the 2019 research, a total of 119 marine species have been found in the Blue Cave, five of which are strictly protected by the law.