A larger sea cave on the western face of the Monte Circeo promontory, notable both as a snorkelling and diving site and as an archaeological site. Fossellone has yielded prehistoric remains — animal bones and evidence of human occupation — and is considered part of the same Palaeolithic landscape as the better-known Grotta Guattari nearby.
The cave is accessible by boat from Porto di Circeo and is included on most of the harbour's cave-tour itineraries. The underwater section near the entrance is a popular spot for snorkellers; scuba divers with local operators can explore the deeper chambers. The cave's name refers to the natural spring (fossellone — "big ditch") that flows through it.