What is another word for underground?

Here are some words that can be used as synonyms for "underground," depending on the context:

General:

* Subterranean

* Subsurface

* Below ground

* Below the surface

* Buried

* Hidden

* Secret

More specific:

* For locations: Basement, cellar, tunnel, cavern, mine

* For activities: Clandestine, covert, secret, illicit, hidden, concealed

Figurative:

* For ideas or movements: Subversive, dissident, rebellious, countercultural, fringe

The best synonym will depend on the specific meaning you are trying to convey.