In 1966, local authorities sealed access to a chalk quarry in Normandy, effectively ending all tourist visits or exploration of its underground passages. Over the years, no archaeological work, construction, or real estate development has justified keeping the site closed. Yet, each winter, as the first frosts arrive, a quiet but massive phenomenon takes place beneath the surface. It is not humans or machinery that fill the space, but thousands of bats seeking a safe place to hibernate. The site is Caumont-sur-Seine, where the history of an abandoned quarry overlaps with one of the most intriguing cohabitations of Norman wildlife. The quarry has been fenced off and designated a regional natural reserve, because any disturbance during hibernation can drain the energy reserves of the bats and lead to their deaths. Though closed in 1966, the site has never truly been abandoned. Before becoming a sanctuary for bats, the Caumont quarry played a significant role in the region’s built heritage. For decades, its underground galleries supplied high-quality limestone used in the construction of major landmarks, including the Rouen Cathedral and the Natural History Museum of Le Havre. However, its fate changed in 1943 during World War II, when the Todt Organization, on behalf of the Nazi regime, requisitioned the site to build military infrastructure. The quarry's proximity to the Seine and its deep, wide galleries made it an ideal location for hidden installations. After the war, mining resumed briefly before ceasing entirely, leaving the site in silence—unaware that it would soon become a refuge for wildlife. What makes Caumont-sur-Seine unique is the presence of thousands of bats that invade the underground galleries every winter. This is not a myth or exaggeration: the conditions in these artificial caves are perfect for hibernation. Stable temperatures, constant humidity, and complete darkness make the quarry an ideal hibernaculum, a term used to describe a protected wintering site. This phenomenon is not unique to Caumont. In Normandy, the Orival quarries in Calvados also serve as a natural reserve for bat hibernation, and in Pas-de-Calais, the Cléty quarry has become a similar refuge. These once-industrial sites have been repurposed by nature into safe havens for endangered bat species. Among the species found in these quarries are the Lesser Horseshoe Bat, the Greater Mouse-eared Bat, and the Edible Bat—some of the most vulnerable bats in Europe. These animals are direct victims of the loss of natural habitats, as hollow trees and old buildings are increasingly rare. Artificial quarries like Caumont have become vital for their survival. The site’s closure since 1966, initially for safety reasons, has unintentionally provided a crucial refuge for these fragile colonies. Recognizing the ecological importance of the site, local authorities have officially protected it, fencing it off to prevent disturbances while allowing bats to use the natural passages they need. This approach has already shown positive results elsewhere in Normandy, with a significant increase in recorded species in similar protected quarries. Behind the scenes, these efforts are quietly transforming former industrial sites into vital sanctuaries for local biodiversity.