RainBoxes are "plastic modular units" (PMU) used to construct underground water storage structures. They are commonly used in Australia for underground detention basins to capture and detain water under parking lots, satisfying water quality requirements. Much like a "concrete masonry unit" (CMU), they may be stacked vertically, horizontally, and aligned in many ways. For water storage and reuse applications the units are wrapped in three layers: geotextile fabric, porous aggregate and impervious liner. They may be easily joined to any pipe size for inlets or outlets, or fitted for maintenance and observation ports.

Constructed applications include box culverts, replacement of drain rock under porous concrete, storage and conveyance under infiltration swales, energy dissipaters at pipe discharge points, water reuse tanks, infiltration tanks and exfiltration tanks.