Abstract: A vault construction is provided for the storage of hazardous or polluting liquids which comprises an inner cylindrical tank for storing such liquids and an outer casing in which the inner cylindrical tank is housed. The outer casing is of two-piece construction and includes a plurality of ribs extending inwardly from the inner surface thereof in engagement with portions of the outer tank wall and defining a plurality of air spaces between the inner wall of the casing and the outer tank wall. The ribs are discontinuous at the bottom of the casing so as to define a channel along the bottom of the casing. Thus, liquid spilled during filling of the tank or liquid leaking from the tank can collect in the channel. The outer casing further includes an indicator device such as a plug of transparent material located in the bottom of the casing, for providing a visual indication that liquid has collected in the channel.