understructure vs foundation

understructure and foundation both are nouns.

Word	Noun	Adjective	Verb	Adverb
understructure	Yes	No	No	No
foundation	Yes	No	No	No

As nouns, understructure and foundation are synonyms defined as:

Other synonyms of understructure include base, foot, fundament, groundwork, substructure.

understructure (noun)	foundation (noun)
lowest support of a structure	the act of starting something for the first time; introducing something new
	lowest support of a structure
	a woman's undergarment worn to give shape to the contours of the body
	the fundamental assumptions from which something is begun or developed or calculated or explained
	education or instruction in the fundamentals of a field of knowledge
	an institution supported by an endowment
	the basis on which something is grounded