The DICT Development Group

  understructure
      n 1: lowest support of a structure; "it was built on a base of
           solid rock"; "he stood at the foot of the tower" [syn:
           foundation, base, fundament, foot, groundwork,
           substructure, understructure]

  41 Moby Thesaurus words for "understructure":
     base, basement, basis, bearing wall, bed, bedding, bedrock, bottom,
     floor, flooring, fond, footing, foundation, fundament, fundamental,
     ground, grounds, groundwork, hardpan, infrastructure, pavement,
     principle, radical, riprap, rock bottom, rudiment, seat, sill,
     solid ground, solid rock, stereobate, stylobate, substratum,
     substruction, substructure, terra firma, underbuilding,
     undercarriage, undergirding, underpinning, understruction