A cubic modular subsurface stormwater unit has equal top and bottom surfaces spaced apart by a set of four pillars, each of which runs from a corner of the top surface to a corresponding corner of the bottom surface. The pillars define a void having a generally cruciform cross section, the void opening onto each of the four side faces defined between the roof and the base. A line of units connected together in series side to side will have a long throughway formed by the respective voids in each unit, and a regular matrix of X- and Y-axis throughways will be formed as units are connected laterally on all sides. The unit is formed of identical halves which can be connected together in any orientation due to the square configuration and to a mating connection which has mating parts symmetrically disposed about a diagonal corner-to-corner line.