foundation vs understructure

foundation and understructure both are nouns.

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

As nouns, foundation and understructure are synonyms defined as:

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

foundation (noun)	understructure (noun)
the act of starting something for the first time; introducing something new	lowest support of a structure
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