Lago di Posta Fibreno is a small natural lake in the municipality of Posta Fibreno, about 4 kilometres east of Broccostella. It is protected as a nature reserve and is notable for its exceptionally clear spring-fed water and the diversity of its aquatic ecosystem — the lake holds one of the last remaining populations of the endangered Fibreno trout (Salmo fibreni), a species found nowhere else in the world.
The floating island — a drifting mass of vegetation that moves around the lake surface — is the feature that draws most visitors, along with the waterfowl that nest along the reed beds. The reserve has a visitor centre and waymarked paths around the lake perimeter. It makes an easy and rewarding excursion from Broccostella or from Sora.