underground
/ˈʌndɚˌgraʊnd/
adjective
located or occurring below the surface of the earth
always used before a noun :secret and usually illegal
always used before a noun :of, relating to, or produced in a social and artistic world that is different and separate from the main part of society
adverb
below the surface of the earth
in or into a place that is hidden or secret :out of the view of the public
noun
Brit :a system of trains that run below the ground in a large city :subway [singular]
the underground :a group of people who secretly work to oppose or overthrow a government