underside vs underbelly

underside and underbelly both are nouns.

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

As nouns, underbelly is a hyponym of underside; that is, underbelly is a word with a more specific, narrower meaning than underside:

Other hyponyms of underside include base, bilge, heel, sole.

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