understructure
1. [ noun ] lowest support of a structure
Synonyms: base foot foundation groundwork substructure fundament
Examples:

"it was built on a base of solid rock" "he stood at the foot of the tower"

Related terms: support raft_foundation bed structure
Similar spelling:   undereducated
  underproduce
  undersecretary
  understated
  undercoated
  undercharge
  understaffed
  underachieve
  underclothes
  underclothed
  underachiever