1. Beneath the surface of the earth.

2. A place or space beneath the surface of the ground.

Nuttall's Standard dictionary of the English language By Nuttall, P.Austin

• undreground,
• undergorund,
• udnerground,
• undergound,
• undergroung,
• undergroun,
• undergrounde,
• undrground,
• underround,
• eunderground,
• undeground,
• undrgroung,
• undergrownd,
• undergrand,
• undergrond,
• unerground,
• undergournd,
• undergrund,
• underegound,
• undergroud

• underdown,
• underbody,
• undergraduate,
• underpin,
• undercoated,
• undersoil,
• underproduce,
• underpopulated,
• undergrowth,
• underclothed,
• underspend,
• underboss,
• underslung,
• underdone,
• underhand,
• undertow,
• underworld,
• undergrow,
• undercharge,
• underfur,
• underwear,
• under control,
• underwing,
• undershot,
• undergird,
• understood,
• underdog,
• undercoat,
• underhung,
• underbodice,
• undertone,
• undersign,
• underpart,
• underhanded,
• underperform,
• undergrad,
• underwood,
• underling,
• undershrub,
• understand,
• underfoot.

The adjective form of the word "underground" is "subterranean".

The plural of the "underground" can be the "undergrounds".

The singular of the "underground" can be the "underground".