The Collaborative International Dictionary of English v.0.48:

Underdrain \Un"der*drain`\, n.
   An underground drain or trench with openings through which
   the water may percolate from the soil or ground above.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Underdrain \Un`der*drain"\, v. t.
   To drain by forming an underdrain or underdrains in; as, to
   underdrain land.
   [1913 Webster]