Riserva naturale Lago di Posta Fibreno, Regional nature reserve in Posta Fibreno, Italy

Riserva naturale Lago di Posta Fibreno is a regional nature reserve in Lazio, built around a small mountain lake fed by underground springs that rise through limestone rock. The water stays cool and clear throughout the year because it comes directly from deep within the ground.

The lake and its surroundings were placed under regional protection in the 1980s after concerns grew about changes to the water quality and the local ecosystem. Since then, the area has been actively managed to keep its natural balance.

The reserve's name comes from the lake it surrounds, and the water plants include Hippuris vulgaris, rare in central Italy and a marker of unpolluted water. On the eastern shore stands the hamlet of Carpello, whose cultivated fields and vegetable plots still reach the lake banks, as farming did across the whole protected area.

Marked paths lead visitors through different parts of the reserve and around the lake shore. Sturdy footwear is a good idea since some trails can be uneven or wet, especially near the water.

A floating island made entirely of water plants drifts slowly across the lake surface, pushed by wind and currents. Ancient Roman writers described this same formation, making it one of the oldest documented natural features of the area.