Ustica Blue Cave provokes emotions every time you enter it, between light and twilight the atmosphere immediately becomes magical. The rinsing of the water that breaks on the rock and the cooing of doves in the niches of the vault create a musical background. The water now seems silver, now a delicate blue; he paints the oar, the bottom of the boat and the friend who swims: boat and man seem suspended in a luminous blue, an enchantment. The cave is accessed from the sea through a large entrance about 9 meters wide, or from the tunnel on the left, about 10 meters long and 4 meters high, submerged for 2 meters, which connects it with the San Francesco Cave. Crossing the entrance you find yourself in a vast hemispherical environment 7 meters high and 30 meters in diameter and a seabed that slopes outwards. From the main room, in addition to the passage of the San Francesco Cave, two other branches branch off which in turn divide the various sections and rooms. The walls of the main hall penetrate the water for 10 meters, a large collapsed boulder is placed at the bottom of the entrance. Everything seems to have been arranged to let the light rays slide between the rocks, in the right intensity and inclination to radiate into the cave, giving life to plays of light and colors that leave you breathless.

The Blue Cave and the San Francesco cave are part of a complex consisting of two caves communicating through an internal tunnel, with two entrances from the sea and one from the ground. The formation of this complex, like all the coastal caves of the island, is due to the infiltration of water mixed with carbon dioxide which over time, in addition to forming stalactites and stalagmites, has a corrosive action that widens the cracks and causes instability and subsequently the collapse of the rocky building giving rise to the formation of the caves. Numerous evidences found inside, in the form of fossil finds, about the marine excursion that took place over the various millennia. Looking at the coast, you only notice the entrance to the Blue Cave, while that of the San Francesco Cave remains shielded by the cliff about twenty meters to the left.