WordNet (r) 3.0 (2006):

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]

Moby Thesaurus II by Grady Ward, 1.0:

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