Since 1930, an entire valley in inland Brittany has been hidden beneath the waters of Lake Guerlédan. This area, once home to lock keepers' houses, stone steps, and a towpath used by generations of workers, now lies dozens of meters below the surface. Every ten years, the dam releases its water for inspection, briefly revealing this submerged landscape to visitors. The lake was created to become the largest in Brittany, but its construction came at a human cost: about 50 families were displaced, and the villages of Saint-Aignan and the Salles-Neuves were partially or fully submerged, forcing their residents to start new lives elsewhere. The dam, a concrete gravity structure, took seven years to build, from 1923 to 1930. It stands 45 meters high above the valley floor and is 206 meters long at its highest point. It holds 55 million cubic meters of water over an area of 304 hectares. The construction also submerged 17 locks on the Nantes to Brest canal, which had been a vital part of the local economy, particularly for slate workers in the region. Time has paused since 1930: trees that once lined the towpath still stand upright, submerged as they were nearly a century ago. The foundations of lock keepers' homes, adjacent gardens, and the mechanisms of the locks remain, waiting for the dam gates to open again. French regulations require that dams over 20 meters in height undergo full inspections every ten years. While the complete draining of the lake doesn’t follow a strict schedule, the last full draining occurred in 2015, with the next expected around 2045. In 1985, the draining event attracted large crowds, causing traffic jams and parking issues. Even thirty years later, the enthusiasm remained strong: in 2015, over two million visitors explored the ghostly valley, marveling at the lunar-like landscape, remnants of lock keepers’ houses, towpaths, and submerged tree trunks. Visitors who walk on the lakebed during these rare dry periods do so under strict conditions. The ground can be unstable, with old mud and fragile remnants posing risks. For safety, guides and marked paths are in place during these exceptional times. The event draws a variety of people—curious hikers and industrial heritage enthusiasts—who come to photograph remnants of a bridge, stone staircase, or a partially collapsed lock keeper’s house. Meanwhile, the Nantes to Brest canal continues to exist on both sides of the dam, serving as a living reminder of the network that the 1930 construction definitively divided in two.