Beneath Montsouris Park in Paris's 14th arrondissement lies a massive underground reservoir built in 1874. This hidden structure can hold up to 202,000 cubic meters of fresh water—equivalent to more than eighty Olympic-sized swimming pools. Constructed during a time of rapid population growth in Paris, the reservoir was designed to manage the city’s water supply more efficiently, balancing usage between day and night. Despite being over 150 years old, it is still in use today, serving the same essential function. Few visitors to the park realize they are walking above this impressive underground space, often referred to as an "underground cathedral." The reservoir features stone vaults supported by massive pillars, creating a grand and imposing interior. This architectural marvel was part of a broader national effort to modernize Paris’s infrastructure, especially in the wake of deadly cholera outbreaks that had exposed the weaknesses in the city's water supply system. Building the reservoir was a major technical challenge at the time. Engineers had to construct watertight vaults capable of withstanding both the weight of the earth above and the pressure of the water stored within. The success of this project marked a significant advancement in civil engineering and urban planning, reflecting the ambition and ingenuity of 19th-century France. Today, the reservoir continues to operate as originally intended, helping to manage fluctuations in water demand and ensuring a continuous supply for Parisians. It is one of many hidden structures that form an "invisible Paris"—a network of underground galleries, quarries, and reservoirs that extend far beyond the city's familiar streets and buildings. These hidden spaces reveal the complex and layered history of Paris, where the past continues to support the present.