subterranean
adjectiveUK /ˌsʌb.təˈreɪ.ni.ən/
US /ˌsʌb.təˈreɪ.ni.ən/
Under the ground.
The ants built a subterranean home beneath the garden.
OTHER MEANINGS
2. Hidden or secret, like something kept out of sight.
The spies used a subterranean network to share messages.
3. Existing deep within a person's mind or feelings.
He had a subterranean fear of the dark that he never talked about.
4. Relating to a place or group that is not well known to most people.
The band was part of a subterranean music scene in the city.