adjective

1. occurring, working, placed, used, etc. beneath the surface of the earth
2. secret; hidden; undercover
3. designating or of newspapers, films, music, etc. that are unconventional, experimental, radical, etc.

adverb

1. beneath the surface of the earth
2. in or into secrecy or hiding; so as to be undercover; surreptitiously

noun

1. the entire region beneath the surface of the earth
2. an underground space or passage
3. a secret movement organized in a country to oppose or overthrow the government in power or enemy forces of occupation
4. an underground movement in media, films, music, etc.
5. Brit. a subway