underworld vs netherworld

underworld and netherworld both are nouns.

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

As nouns, underworld and netherworld are synonyms defined as:

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

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