understructure vs substructure

understructure and substructure both are nouns.

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

As nouns, understructure and substructure are synonyms defined as:

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

understructure (noun)	substructure (noun)
lowest support of a structure	lowest support of a structure
	the basic structure or features of a system or organization