The “Posta Fibreno Lake” Nature Reserve was established in 1983 and falls entirely within the territory of the municipality of Posta Fibreno (FR), which manages it. The lake has no tributaries because it originates from a system of karstic springs that derive from the catchment basin of the upper Sangro Valley in Abruzzo and has only one emissary, the Fibreno River from which it derives its name. Its waters have an almost constant temperature throughout the year of around 10-11 °C. The lake has a length of about 1096 m and a maximum width of 570 m, with an average depth of 2.5 m (the maximum is 15 m).

It has a characteristic feature, perhaps unique in Europe: the ‘Rota’, a floating island about 30 m wide made up of peat, roots and rhizomes, which moves with the wind or with the increased flow of nearby springs.

The main watercourse of the basin, the Fibreno river, originates from the confluence of the waters of the Posta Fibreno lake with those of the Carpello stream. It has a flow rate of about 6 m3/s and follows a winding course for about 11 km until the confluence with the Liri river.

Since 1977, a Cross has been placed in the Codigliane locality, where the lake reaches its maximum depth, to protect the water and those who live there. The work, made of steel and platinum by the late sculptor Pino Bonavenia, is brought to the surface every year during the ‘Crucifix and Diver Festival’, celebrated every first weekend in August.

On Saturday evening, the Cross is carried in procession over the lake, along a route marked by torches and accompanied by a procession of boats and canoes. The following morning, immediately after the Holy Mass, divers from different regions of Italy dive to place the Cross back on the bottom of the lake, so that it can protect the inhabitants of Posta Fibreno for another year.

The ‘nauè’ is a small local boat made of oak wood, with a flat bottom and two prows, which can carry about 3-4 quintals; a model is preserved in the Ethnographic Naturalistic Museum in the historic centre of Posta Fibreno.

Also worth a visit the Mill built in 1810 in the locality of Sorgentina, now used as the Reserve centre. Four suggestive nature trails: “Catannovo”, “Lago Chiaro”, “Dolina la Prece” and “Puzzillo” offer tourists the opportunity to discover and admire the beauty of the Natural Reserve.

Protected animal species include the little egret, great egret, ferruginous heron, ferruginous duck, pochard, teal, shoveler, cormorant and grey heron, which stop on the shores of the lake during migration.

Among the more stationary species of aquatic fauna are the coot, moorhen and mallard. Migratory birds include the grey heron, cormorant, shoveler and teal.

The most important species in the Reserve is a fish that only lives here: the ‘Fibreno carp’.