belowground vs underground

belowground is not a noun while underground is a noun.

Word	Noun	Adjective	Verb	Adverb
belowground	No	Yes	No	No
underground	Yes	Yes	No	Yes

As adjectives, belowground and underground are synonyms defined as:

belowground (noun)	underground (noun)
	an electric railway operating below the surface of the ground (usually in a city)
	a secret group organized to overthrow a government or occupation force

belowground (adjective)	underground (adjective)
underneath the ground	conducted with or marked by hidden aims or methods
under the level of the ground	under the level of the ground

belowground (adverb)	underground (adverb)
	beneath the surface of the earth
	in or into hiding or secret operation