In ancient times the island Ponza was called Tyrrhenia. Legend says that Ponza is what is left of the lost island of Tyrrhenia. Ponza is said to have been connected to the mainland by a narrow strip of land, which sank into the sea with most of the island below the water, with the parts remaining above sea level. (Wikipedia) Read more about Ponza
The trip to Ponza from Terracina takes about an hour by the fast ferry. Tickets can be bought at the docks. At Ponza you can also go on excursions by smaller boats around the island. On these trips you can enjoy breathtaking scenery and crystal clear water.