bottom vs underside

bottom and underside both are nouns.

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

As nouns, bottom and underside are synonyms defined as:

Other synonyms of bottom include undersurface.

bottom (noun)	underside (noun)
a cargo ship	the lower side of anything
the fleshy part of the human body that you sit on
the lower side of anything
the lowest part of anything
a depression forming the ground under a body of water
low-lying alluvial land near a river
the second half of an inning; while the home team is at bat

bottom (adjective)	underside (adjective)
situated at the bottom or lowest position
the lowest rank

bottom (verb)	underside (verb)
come to understand
strike the ground, as with a ship's bottom
provide with a bottom or a seat