underworld vs stratum

underworld and stratum both are nouns.

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

As nouns, stratum is a hypernym of underworld; that is, stratum is a word with a broader meaning than underworld:

Other hypernyms of underworld include class, social class, socio-economic class.

underworld (noun)	stratum (noun)
(religion) the world of the dead	an abstract place usually conceived as having depth
the criminal class	a subpopulation divided into a stratified sampling
	people having the same social, economic, or educational status
	one of several parallel layers of material arranged one on top of another (such as a layer of tissue or cells in an organism or a layer of sedimentary rock)