undersoil vs dirt

undersoil and dirt both are nouns.

Word	Noun	Adjective	Verb	Adverb
undersoil	Yes	No	No	No
dirt	Yes	Yes	No	No

As nouns, dirt is a hypernym of undersoil; that is, dirt is a word with a broader meaning than undersoil:

undersoil: the layer of soil between the topsoil and bedrock
dirt: the part of the earth's surface consisting of humus and disintegrated rock

Other hypernyms of undersoil include soil.

undersoil (noun)	dirt (noun)
the layer of soil between the topsoil and bedrock	disgraceful gossip about the private lives of other people
	anything regarded as making something unclean
	the part of the earth's surface consisting of humus and disintegrated rock
	obscene terms for feces

undersoil (adjective)	dirt (adjective)
	(of roads) not leveled or drained; unsuitable for all year travel