The Ouso Passo Pratiglio is the deepest cave of Lazio that starts at an altitude of 1,360 meters on the south-east of Mount Malaina, in the Lepini Mountains, and has a vertical drop of about 840 meters.

The explorations were arrested in front of a large lake-siphon which receives water from a large funnel. This, together with the strong air circulation, suggests the presence of another entrance on top of the mountain yet to be identified.