A modular storm water retention system for use in constructing an underground storm water retention structure, the storm water retention system comprising: - a plurality of modular fluid retention units , each retention unit comprising: a bottom portion defining at least a first and a second opening ; a top portion connected to the bottom portion , the top portion having a substantially horizontal support surface , the top and bottom portions defining an interior water retention chamber volume beneath the top portion in communication with the first and the second openings defining a first through passageway between the first and the second opening; - a plurality of modular trays positioned above the retention unit top portion and selectively engageable with the top portion substantially horizontal support surface , the respective modular trays each having peripheral sides and a top surface , the respective modular trays positioned adjacent to one another having one peripheral side in close proximity to an adjacent modular tray peripheral side, each modular tray top surface extends between at least two adjacent retention units and is supported by the respective retention unit substantially horizontal surface , the respective modular trays substantially covering interstitial volume spaces between retention units below the respective modular trays preventing backfill material from entering the interstitial volume spaces , said system being characterized in that each retention unit has a connector portion adapted to be connected to a connector portion of an adjacent retention unit to extend the first through passageway between connected retention units through the respective first and second openings and define an enclosed increased interior chamber volume. - a plurality of modular fluid retention units , each retention unit comprising: a bottom portion defining at least a first and a second opening ; a top portion connected to the bottom portion , the top portion having a substantially horizontal support surface , the top and bottom portions defining an interior water retention chamber volume beneath the top portion in communication with the first and the second openings defining a first through passageway between the first and the second opening; a bottom portion defining at least a first and a second opening ; a top portion connected to the bottom portion , the top portion having a substantially horizontal support surface , the top and bottom portions defining an interior water retention chamber volume beneath the top portion in communication with the first and the second openings defining a first through passageway between the first and the second opening; - a plurality of modular trays positioned above the retention unit top portion and selectively engageable with the top portion substantially horizontal support surface , the respective modular trays each having peripheral sides and a top surface , the respective modular trays positioned adjacent to one another having one peripheral side in close proximity to an adjacent modular tray peripheral side, each modular tray top surface extends between at least two adjacent retention units and is supported by the respective retention unit substantially horizontal surface , the respective modular trays substantially covering interstitial volume spaces between retention units below the respective modular trays preventing backfill material from entering the interstitial volume spaces , <EOS>