A modular raintank and water storage system are described. A modular raintank comprises a plurality of interconnected external sidewall modules. The sidewall modules have a plurality of openings which allow water to freely flow into and out of the modular raintank. The water storage system comprises a plurality of interconnected modular raintanks. The adjacent modular raintanks of the water storage system can share a sidewall, and can be stacked on top of each other and connected in a side-by-side pattern. The shared sidewall comprises a plurality of locking lip members arranged in rows to facilitate the attachment of additional modular plates of adjacent raintanks and a plurality of U-shaped openings to facilitate visual inspection of the tank while underground.