On November 3, 2019, at dawn, a dozen volunteers stationed around Lake Der counted 268,120 grus leaving the basin simultaneously. According to LPO Champagne-Ardenne, which organizes these counts every autumn, this is the world record for the concentration of birds stationary at a single site. This number far exceeds the usual scores for the season, which are typically between 50,000 and 90,000 individuals at the peak of the migratory season. On that day, nothing had been planned in advance. The grus had accumulated further north, blocked by capricious weather, before departing all at once as soon as an anticyclone settled. Result: a record of 268,120 grus, nearly the entire population of wintering birds in Spain concentrated on this single Champagne lake for the duration of a stopover. Five years earlier, in November 2014, a previous record had already been set. The LPO Champagne-Ardenne team had organized an exceptional count at the last minute after observing the massive arrival of migratory flights coming from Germany and Belgium, which allowed them to count 206,582 grus, a European record for birds stationary at a single site. The 2019 score shattered this previous maximum with a spectacular increase in numbers. Lake Der hosted 268,120 grus on November 3, 2019, breaking the world record for the concentration of birds on a single site. This 48 km² water body is entirely artificial, constructed in 1974 to regulate the floods of the Marne. The autumn migratory peak usually peaks in November, with about 300,000 grus passing through the site each year. A lake that did not exist fifty years ago. Here is the paradox: this flock of birds lands on an entirely artificial body of water. Lake Der was created in 1974 after ten years of massive work, designed mainly as a reservoir to regulate the water levels of the Marne river, which feeds the Seine. In 1976, exactly fifty years ago, the water body had existed for only two years. Its primary function is not natural. The Marne reservoir, known as Lake Der-Chantecoq, aims to strengthen the flow of the Marne during low water periods and reduce the magnitude of floods. It is named after the village of Chantecoq, which was destroyed, along with Champaubert-aux-Bois and Nuisement-aux-Bois, during the construction work. Three villages submerged to protect Paris from floods. The industrial scar has become a sanctuary. With its 48 km², Lake Der remains the largest artificial lake in metropolitan France by surface area, capable of storing 350 million cubic meters of water. Around its 77 kilometers of shores, a mosaic of wet meadows and shallow waters has developed, offering the grus a secure resting place in the water, away from terrestrial predators. An avian magnet that has become indispensable. The scale of the phenomenon has developed over a few decades. In 1977, 40,000 grus passed through Lake Der, and today nearly 300,000 migrate through this site. A seven- or eightfold increase in air traffic, without any human development having been planned for this purpose from the start. The mechanism is simple. During the day, the grus leave the lake to feed in the surrounding fields, particularly in leftover corn after harvests. At night, they return to sleep on the water, standing in the shallow areas, secure. The autumn passage begins in October and usually peaks in November. Counts conducted since 2017 by LPO Champagne-Ardenne show that the peak never occurs before mid-October or after November 20. A weekend at the end of October or beginning of November remains the most promising period for observing this dance.