Lac de Chalain, Natural lake in Fontenu, France

Lac de Chalain is a 232-hectare glacial lake in the Jura, 2,700 m long and 1,100 m wide, with a maximum depth of 32 m and an average depth of 16.2 m. Its rectangular basin lies in a reculée framed by 60–80 m limestone cliffs, with wooded slopes on three sides and a floodplain on the east shore. The Bief de l'Œuf, a 2 km outflow channel, drains the lake toward the Ain.

Archaeological excavations starting in 1904 uncovered a lakeside settlement from 5,000 years ago along the western shore, consisting of houses built on stilts. Further research during the 1970s and 1980s documented multiple construction phases between the fourth and third millennia BCE, revealing remains of tools and pottery.

The site and its prehistoric pile dwellings form part of a UNESCO World Heritage designation documenting early Alpine settlement from around 5,000 years ago. These Neolithic structures provide insight into how the first sedentary communities lived along the shoreline, demonstrating advanced construction techniques using wooden poles driven into shallow waters.

Two supervised beaches offer swimming access during summer months, with parking fees in effect from June through September. Several hiking trails circle the shoreline and lead to viewpoints, while boat rentals and campsites operate near the villages of Doucier and Marigny.

The lake has no notable surface inflow: its water arrives underground, having seeped through the karst plateau from the small lakes of Narlay and Vernois about 10 km east, a link proven by dye-tracing tests. Despite this, the water is renewed roughly every ten months. The outflow, the Bief de l'Œuf near Marigny, drives a hydroelectric turbine.