An artificial lake known as Lake Der, spanning 48 square kilometers, is the largest in metropolitan France by surface area. Since 1974, this vast reservoir has submerged the exact location of three villages in the Champagne region—Chantecoq, Champaubert-aux-Bois, and Nuisement-aux-Bois. Nearly 300 residents were displaced to create the lake, which was designed to protect Paris from flooding. The reservoir, known as Lake Der-Chantecoq, was built to manage flood risks from the Marne River, a major tributary of the Seine. The lake can store up to 350 million cubic meters of water and empties each autumn to release water back into the Marne, maintaining a natural flow and preventing downstream flooding. The decision to build Lake Der came after two major flood events in 1910 and 1924, when the Seine and its tributaries overflowed, submerging parts of Paris. These disasters prompted the government to rethink flood management, particularly for the Marne, which flows 190 kilometers downstream from Paris. Construction of the reservoir began in 1967 and took six years to complete. The project involved transporting millions of cubic meters of clay, rocks, and concrete, as well as building 14 bridges, 25 kilometers of roads, and 24 kilometers of dikes. The lake was officially inaugurated on January 3, 1974, after the three villages were completely submerged. The three villages were entirely erased from the landscape to make way for the reservoir. Champaubert-aux-Bois, the most populous of the three with 225 residents, was followed by Nuisement-aux-Bois and Chantecoq, which had 63 and 57 residents respectively. The homes of these villages were not just abandoned—they were destroyed and removed, leaving only the vast expanse of water. Today, the memory of these lost communities is preserved in a museum in Nuisement-aux-Bois, where wooden-framed houses saved from the water have been reconstructed. The villages’ names were also commemorated in Vitry-le-François, where streets are named after them to mark the 50th anniversary of their disappearance. Only one structure from the submerged villages remains above water: the Church of Saint-Laurent in Champaubert. Built in the 16th century and rebuilt in the 1870s, the church was slightly elevated to avoid submersion. It now stands alone in the middle of the lake, becoming a symbolic landmark. Each year, as Lake Der empties in the autumn, the surrounding landscape transforms. Mudflats appear, and the church’s foundations become visible, offering a glimpse into the past. This seasonal cycle of filling and draining is essential to the lake’s function—storing floodwaters in winter and releasing them in summer to maintain the Marne’s flow and protect Paris from potential floods.