Located or operating beneath the earth's surface
A clandestine organization of freedom fighters in an oppressed land
He breathed deeply, not realizing how musty the underground world was until he breathed fresh air.
Take her underground, if you must.
She followed her friend to a portion of the underground site converted into a massive gym and training facility.
He stopped outside the door to the underground dungeon, his skin crawling at the scent of earth all around him.
Move them to the nearest underground site today.