existing, situated, or operating below the surface of the earth; underground.

existing or operating out of sight or secretly; hidden or secret.

a person or thing that is subterranean.

a subterrane.

adjective

below ground , buried , covered , covert , hush-hush * , on the qt , private , secret , subterrestrial , subversive , sunken , underfoot , under wraps * , hypogeal , hypogean , hypogeous , cave , cavern , concealed , hidden , underground