substructure vs understructure

substructure and understructure both are nouns.

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

As nouns, substructure and understructure are synonyms defined as:

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

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