An underground infiltration system consists of open-bottomed storage chambers in a crushed stone reservoir. The chamber and crushed stone reservoir provide temporary storage for stormwater before it infiltrates into the underlying soil. A number of underground infiltration chamber products, including pipes, vaults, and modular structures, have been developed as alternatives to infiltration trenches and basins for space-limited sites and stormwater retrofit applications. Similar to traditional infiltration trenches and basins, these systems are designed to capture, temporarily store, and infiltrate stormwater runoff. Underground infiltration systems are typically designed as off-line systems for retention/runoff reduction, treatment, and groundwater recharge. These systems can provide stormwater quantity control for larger storms when used in soils with high infiltration rates or when designed with additional below-ground storage. The design and layout of these systems varies by manufacturer and system design.

While underground infiltration systems are more costly than other Infiltration BMPs that are located at the surface, they can be an effective approach to manage stormwater where there is little or no space on the surface.