[1] : WordNet (r) 2.0

understructure
     n : 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
see also:
foundation base fundament foot groundwork substructure 

[2] : 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

Understructure >> U Words
Understructure, definition of term: Understructure
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