netherworld vs underworld

netherworld and underworld both are nouns.

Word	Noun	Adjective	Verb	Adverb
netherworld	Yes	No	No	No
underworld	Yes	No	No	No

As nouns, netherworld and underworld are synonyms defined as:

Other synonyms of netherworld include Hades, Hell, infernal region, Scheol.

netherworld (noun)	underworld (noun)
(religion) the world of the dead	(religion) the world of the dead
	the criminal class