Réservoir de Montsouris, Underground water reservoir in 14th arrondissement, France

The Réservoir de Montsouris is a covered reservoir on a rise near Parc Montsouris, in the 14th arrondissement. Two pairs of superimposed basins, 265 m by 135 m, lie under three hectares of lawn that keep the water at a constant 12 °C. The upper basins are 3.30 m deep, the lower ones over 5 m. A lantern tower with a ceramic ceiling by Janin Frères and Guérineau tops the structure.

Built between 1869 and 1874 by the engineer Eugène Belgrand, the reservoir was delayed by the Franco-Prussian War and the Commune. It was first called réservoir de la Vanne, then de Montrouge. Its water reached the Champs-Élysées in 1875. Built over old quarries, it required nearly 1,800 masonry pillars and five consolidated sinkholes, supervised by Octave Keller.

The reservoir's name comes from the neighbouring Parc Montsouris, though it was first called réservoir de la Vanne, then réservoir de Montrouge. Its lantern roof was decorated by Janin Frères and Guérineau of the Paris ceramic manufactory with the city's coat of arms and the names of the feeding sources — Vanne (1874), Loing and Lunain (1900), Voulzie (1925). The site is closed to visitors; the grass-covered butte above the tanks is what passers-by see.

The reservoir is a working water facility, not a museum: it sits inside a closed quadrilateral bounded by avenue Reille to the south, rue de la Tombe-Issoire to the west and rue Saint-Yves to the north and east, and there is no public entrance. The grass-covered roof, three hectares of lawn, can be seen from the surrounding streets and from the nearby Parc Montsouris.

Before modern electronics, the operators bred trout in tanks to check water quality, as the fish served as natural indicators of contamination. This simple but effective method was once the best way to monitor whether the water was safe to drink.