At an altitude of 350 meters in the Montagne Noire, Lake Saint-Ferréol covers 67 hectares and holds about 6 million cubic meters of water. The lake was built between 1667 and 1672 to provide water for the Canal du Midi, a major engineering project in southern France. The dam that holds the lake is thick rather than tall, and it is supported by embankments on both the upstream and downstream sides, which help it resist the pressure of the water. The lake is located 35 kilometers from Naurouze, a key point in the canal system. The highest point of the route, the Naurouze threshold, is at 189 meters, and the water from the lake was intended to reach this location to supply the canal. Pierre-Paul Riquet, the engineer behind the canal, died in 1680, a few months before the canal's inauguration in 1681. Soon after the canal opened, it became clear that the original reservoir was too small. Riquet had initially planned for a reservoir holding 3.5 million cubic meters, but after an inspection in 1685, military engineer Sébastien Le Prestre de Vauban recommended expanding the dam. This expansion increased the lake's capacity to 6 million cubic meters, a volume that remains today. The site has been recognized as a historical monument since 1997 and continues to serve its original purpose of supplying water to the Canal du Midi. Water from the lake takes about ten hours to travel through underground galleries and ditches to reach the Naurouze threshold. Riquet's plan involved capturing water from the rivers in the Montagne Noire, but these rivers experience low flow during dry periods. To address this, he designed a large dam on the Laudot River in the Saint-Ferréol valley. The dam was completed in 1672 and filled with water in 1675. Inside the dam, a gallery provides access to a valve room where three bronze valves once regulated the flow of water to the canal. Water exits the lake through a lower gallery known as the drainage vault, eventually reaching the canal via the Vauban Plain ditch. This ditch, which winds through the Lauragais region, was extended by Riquet from an older water intake system dating back to the 14th century. The dam is occasionally drained to inspect its structure, a process that has occurred several times, including in 2004 and between 2016 and 2017. Draining the lake reveals the dam's vast size and creates a lunar-like landscape beneath the water. During these inspections, the valves release a powerful jet of water, and a pyramid near the dam, used to measure water levels, becomes visible. These periodic inspections ensure the continued functionality of the dam, which remains a vital part of the Canal du Midi system.