A unique karstic system joins the two cavities of the Poesia Piccola and the Poesia Grande in a partially underwater complex consisting of three elliptical halls and numerous tunnels. The caves follow a circular patten and have two entries: one by sea at the base of the cliff and one by land in where the vault of the main hall collapsed.
The cave of the Poesia Grande is joined by a brief siphon to a cavern with a sandy beach:  from here, another brief siphoned passage leads to the cave of the Poesia Piccola. The latter, in particular, plays an important role due to the numerous votive inscriptions, signs and symbols that date between the second millennium B.C. and the era of the Roman Republic.
The name of these caves comes from the medieval Greek work 'posià', indicating a place where it was possible to drink fresh water thanks to the presence of a spring.