A river known as the Bièvre, stretching 35 kilometers, flows silently beneath Paris, hidden from view since the early 20th century. Once a natural waterway, it was buried under the city for public health reasons and is now part of the city's underground sanitation system. The Bièvre begins in the Yvelines region, near the towns of Bouviers and Guyancourt, and historically connected to the Seine near Austerlitz before being covered. It was heavily polluted in the past by tanneries, dye factories, and laundries, prompting efforts to channel it underground between 1828 and 1912. This work was completed in 1912 as part of large-scale urban improvements inspired by the Haussmannian reforms, which reshaped much of Paris in the 19th century. Despite being hidden, the Bièvre's presence is still visible in parts of Paris. Street names like the rue de la Fontaine-à-Mulard, the rue du Moulin-des-Prés, and the rue de Croulebarbe in the 13th arrondissement reflect the river’s historical significance. Some sidewalks in the neighborhood feature small medallions that mark where the river once flowed. Outside of Paris, about 20 kilometers of the river remain above ground, while 11 kilometers are covered by slabs beyond the city’s limits. In 2007, the Bièvre was officially recognized as a river again, sparking restoration efforts in the surrounding suburbs. One notable project involved reopening a section of the river in the Parc des Prés de la Bièvre in Fresnes in 2003. Another project resurfaced 600 meters of the river between the towns of Arcueil and Gentilly, which required a substantial investment of 10 million euros. A 2001 study proposed bringing back approximately 2,300 meters of the river within Paris, particularly near Kellermann Park and the Muséum national d'Histoire naturelle. However, such ambitious projects are rare due to their high costs and logistical challenges. The revival of the Bièvre represents a broader movement to reconnect with natural elements in urban environments. While the river remains largely hidden beneath Paris, its restoration in certain areas has brought renewed attention to its ecological and historical value. These efforts highlight the ongoing balance between urban development and environmental preservation in one of the world's most iconic cities.