subsoil vs undersoil

subsoil and undersoil both are nouns.

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

As nouns, subsoil and undersoil are synonyms defined as:

subsoil and undersoil: the layer of soil between the topsoil and bedrock

subsoil (noun)	undersoil (noun)
the layer of soil between the topsoil and bedrock	the layer of soil between the topsoil and bedrock