" The opera house needed a much deeper basement in the substage area than other building types, but the level of the groundwater was unexpectedly high. Wells were sunk in February 1862 and eight steam pumps installed in March, but despite operating continuously 24 hours a day, the site would not dry up. To deal with this problem Garnier designed a double foundation to protect the superstructure from moisture. It incorporated a water course and an enormous concrete cistern (cuve) which would both relieve the pressure of the external groundwater on the basement walls and serve as a reservoir in case of fire. " (WIkipedia)