There are more than a thousand submerged caves, pits and submarine freshwater springs along the Croatian part of the Adriatic coast. These karstic phenomena are hot spots for experienced and technical SCUBA divers and visiting them is offered by numerous diving centres. However, very few of these centres can provide their guests with information about the organisms living inside. The caves are inhabited exclusively by animals since there is no light inside. A “biocenosis of semi dark caves” is present in the frontal parts of the caves. There are many sponges, cnidarians, molluscs and bryozoans. In contrast, the inner parts of the caves have almost “naked” walls, with few species of sponges, bryozoans, polychaetes and brachiopods - a “biocenosis of completely dark caves”. On the other hand, structures like “cold-water marine pits” sometimes house unusual carnivorous and glass sponges. Also, the vagile fauna is interesting: mysids, cave shrimps, and dark-dwelling fish.