underbody vs underbelly

underbody and underbelly both are nouns.

Word	Noun	Adjective	Verb	Adverb
underbody	Yes	No	No	No
underbelly	Yes	No	No	No

As nouns, underbody and underbelly are synonyms defined as:

underbody and underbelly: the soft belly or underside of an animal's body

underbody (noun)	underbelly (noun)
the soft belly or underside of an animal's body	the quality of being weak or unprotected
	the soft belly or underside of an animal's body
	lower side