Posta Fibreno is a karstic lake in the foothills of Italy's Lazio region, and though it may seem more like an engrossed river, it is a small, serene lake home to a very unusual natural phenomenon known since Roman times. Described by Pliny the Elder in his 1st-century Naturalis Historia, the lake is the setting for the subtle dances and movements of a natural floating island.

The island, known as La Rota for its round shape, was formed over the years by free-floating rhizomes, peat, and roots. At about 13 feet thick and 130 feet wide, the mass is not rooted to the ground below the water, and thus the wind and currents cause it to move freely around the surface of the lake. Small trees and shrubs cover the land, and when a breeze blows or the volume of the water released by the underwater springs varies, the island appears to be somehow floating around on its own. This is a phenomenon so rare that Posta Fibreno is one of the only places on Earth to see it occur.

Geological anomaly aside, Lake Posta Fibreno's mild temperature and extraordinarily clear waters have made it a paradise for divers, kayakers, wildlife lovers, and birdwatchers. The landscape is a nature reserve that protects a local endemic species of fish, the Fibreno trout. The lake has no tributaries and it is fed by karstic springs, whose constant flow are responsible for the turquoise water's remarkable transparent hue. The fishermen on Posta Fibreno still use a traditional boat known locally as a "nàue," and these may be spotted when walking along the nature reserve's boardwalks.