A modular assembly is provided for detaining or retaining a liquid beneath a ground surface. The assembly featuring a plurality of substantially identical precast unitary concrete modules with each module having a deck portion with outer edges and a plurality of spaced apart legs extending from the deck portion. At least two of the legs of a module are spaced inwardly from the nearest outer edge of the deck portion of the same module so that a portions of the deck overhangs each of said legs. The legs and deck portions define flow paths within each module that are in fluid communication with each other with the flow paths being both outside the legs and between the legs. Each module in the assembly is further positioned with its deck adjacent to the deck of another module.