This sea cave with the magnificent 27 metres tall entrance looks like a passage to another world. The Poseidon’s gate leads into the longest semi-submerged sea cave on the eastern side of the Adriatic Sea (160 m). It used to be a home to the only Mediterranean seal, the Mediterranean monk seal (Monachus monachus), after which it was named. Monk seals no longer visit the cave, but the return of the Mediterranean monk seal into the Adriatic Sea is a matter that must not be disregarded, especially having in mind that its population still exists in the nearby Aegean Sea. Having that in mind, the organised visits will take into account the preservation of this valuable habitat. Light sources may not be used in the cave without permission. The same goes for making noise and visiting the small beach at the far end of the cave that was once used by the monk seal to raise their young. This cave, in some places completely dark, is also a habitat of a series of other marine organisms. It is especially important for diurnal migration of species that hide in the dark during the daylight. In the atrium of the cave a big school of sand smelt (Atherina boyeri) can be found, while numerous mysid shrimps of the Hemimysis genus can be found further inside the cave. The rocks are covered by numerous bryozoans, sponges and polychetes of various colours and strange forms and textures. Strictly protected species the purple sea star (Ophidiaster ophidianus) and the cowry (Luria lurida), as well as the photogenic golden coral shrimp (Stenopus spinosus) can be spotted on the rocks.