On the island nation of Singapore land is at a premium, so a planned petrochemical storage facility is being constructed 150 meters under the sea floor. The article does not give the total underground storage volume, but they do say that an equivalent above-ground facility would require 70 hectares (170+ acres) of land. Now only 10 hectares are required. Check out this tidbit about some of the technology required for the facility:
New technologies are also being used to prevent potential accidents, such as oil seeping through the rock. Part of this involves the use of a “water curtain” – water-filled tunnels and boreholes that will surround the cavern from the outside – in order to keep it sealed through hydrostatic pressure.