In the old slate quarries of Trélazé, near Angers in western France, a striking blue-green water covers the remnants of more than six centuries of mining activity. These quarries, once the largest in the country, were closed in 2014. The unusual color of the water comes from minerals in the slate rock that release fine particles into the water, creating an almost surreal appearance. This visual effect is harmless in itself, but the area remains dangerous for visitors. Slate extraction in Trélazé began in the 15th century, with the first open-pit quarry opening in 1406. For centuries, workers used hand tools and manually operated pumps to extract the slate. The 19th century brought industrial changes, with steam engines allowing deeper and more efficient extraction. The quarries were once a major industrial site, but today, the landscape is a mix of dry land and water-filled basins, known as "bas-fonds," which reflect the area's industrial past. The Trélazé quarries have been redeveloped into a natural park, covering 200 hectares of slatey terrain where vegetation has taken over. However, the area is strictly off-limits for swimming. Local authorities and the managing body, Angers Loire Métropole, warn that the basins are extremely hazardous. The risk comes from the depth of the water, the icy temperatures, and the steep, uneven access points. A sudden drop into the cold water can cause hydrocution, a dangerous reaction where the body is stunned by the temperature shock, even in strong swimmers. This danger is not unique to Trélazé. In northern France, a similar site called "Blue Water" in Trélon has also seen fatal incidents, with the last drowning recorded in 2006. Despite the risks, the area has been recognized for its ecological value and is now a protected natural site. Diving clubs use it for training, sometimes placing old vehicles in the water to simulate challenging conditions. However, the site's allure continues to grow, with online interest in the blue-green basins increasing by nearly 195% over the past summers, largely fueled by social media posts that often ignore the strict safety warnings.