Moulins souterrains du Col-des-Roches, Underground watermill and technology museum in Le Locle, Switzerland.

The Moulins souterrains du Col-des-Roches are underground mills in a limestone cave at the Col des Roches, 2 km west of Le Locle on the French border. Inside, a system of water wheels arranged one above the other drove mills, threshing machines and a sawmill. The Bied originally flowed into the cave and disappeared underground; today it runs through a canal in the mountain, and water for the mills is pumped back up.

In 1652 the Council of State of Neuchâtel granted Isaac Vuagnieux, Balthazard Calame and Daniel Renaud a concession to use the Bied's current, and they built a mill with two wheels. Jonas Sandoz enlarged the site from 1660; Jean-Georges Eberlé modernised it in 1844. The commune bought it in 1884 and converted it into a slaughterhouse in 1898, closed in 1966.

The site is run by the Confrérie des Meuniers du Col-des-Roches, a brotherhood of millers and cave enthusiasts who began clearing slaughterhouse waste from the cave in 1973. A museum above the mills, opened with a foundation created in 1992, explains milling, sawing and power generation, and since 2001 a permanent exhibition titled From Grain to Bread covers cereal growing, milling and baking.

Access is by guided tour available from March through November. Wear comfortable shoes and be prepared for stairs and damp conditions underground.

The machines visitors see running are restored original equipment, not reconstructions: after the site closed in 1966, the Confrérie des Meuniers du Col-des-Roches began clearing slaughterhouse waste from the cave in 1973, and since 1 July 1987 several of the old machines have worked again for the public.