dry well

Short definition from Wiktionary

Noun

1. 1
   An underground structure (for example, a pit filled with gravel) that disposes of unwanted water (most commonly surface runoff and stormwater and in some cases greywater) by holding it and dissipating it into the groundwater.
2. 2
   Used other than figuratively or idiomatically: see dry, well. Any kind of well that is dry.