Beneath the Cochin Hospital lies part of the vast quarry system, excavated in the Middle Ages to provide Paris with building stone. The site was originally a Carthusian Monastery, with the hospital developing from the 17th century. Apart from the Catacombs museum site, most of Paris’s quaryy network is nor publically accessible but the Cochin Hospital site is looked after by SEADACCC (Societe d’Etude et d’Amenagement Des Anciennes Carrieres des Capucins) who open it by arrangement for group visits.
Today accessed by a staircase of over 100 steps, the quarry galleries are around 20 metres deep and extend to around a kilometre. . There are artefacts on display and the walls show evidence of carving, both from the period of extraction and of the later consolidation work. One fascinating feature is a set of steps which descend down beneath the water table. This was primarily built to monitor the level of the ground water, rather than for extraction purposes.