In a system and method for retaining or detaining a fluid beneath a ground surface, a plurality of modules, each having at least one vertically disposed side portion supporting a horizontally disposed roof are provided. The plurality of modules, each being in fluid communication, either directly or indirectly, with each of the other modules, are arranged in an assembly having a plurality of rows and columns. Each of the rows and columns contain at least one flow obstructer such that fluid flow through each of the rows and columns is circuitous.