1. beneath the surface of the ground: traveling underground by subway.
2. in concealment or secrecy; not openly: subversion carried on underground. adjective
3. existing, situated, operating, or taking place beneath the surface of the ground.
4. used, or for use, underground.
5. hidden or secret; not open: underground political activities.
6. published or produced by political or social radicals or nonconformists: an underground newspaper.
7. avant-garde; experimental:
"people can be underground in pits."
"hills can be underground after years."
"workers can be underground for times."
"toads can be underground for periods."
"societies can be underground as movements."
This list can help you learn new vocabulary more quickly and be able explore other words more easily.