Triple walled underground storage tank

A triple-walled underground storage tank is provided with primary, secondary and tertiary walls. Between each two adjacent walls, an annular space is provided. The annular space is continuous throughout, and permits the flow of liquid therein. Monitors may provided separately for each annulus, to detect the formation of leaks, or a lack of containment, in either wall defining the annular space. In one embodiment, the tertiary wall is a smooth cylinder formed about the circumferential ribs of the secondary wall. In the alternative embodiment, the rib-bearing wall is the tertiary wall, and the walls and annular materials of the tank are formed within a female mold. In either case, a tank prepared from integral ribs, with superior robustness and strength, is provided with triple containment.