Underground describes something located beneath the earth's surface. A subway runs underground through tunnels below city streets. Coal miners work underground, descending deep into the earth to extract minerals. Prairie dogs live in underground burrows, creating vast networks of tunnels and chambers beneath grasslands.

The word also describes things that operate in secret, outside official systems or laws. During World War II, resistance fighters formed underground networks to oppose Nazi occupation, hiding people and smuggling information. Before the Civil War, the Underground Railroad helped enslaved people escape to freedom through a secret system of safe houses and guides. An underground newspaper might publish controversial ideas that mainstream media won't print.

In culture, underground refers to art, music, or movements that exist outside the mainstream. Underground hip-hop artists might record in home studios and distribute their music independently, building audiences without radio play or major record labels. Underground comics push boundaries that conventional publishers avoid.

The metaphor makes sense: just as physical underground spaces are hidden from view, underground activities and movements operate beneath the surface of ordinary society, sometimes by choice and sometimes by necessity. Whether it's literal tunnels or secret organizations, underground suggests something concealed, alternative, or operating beyond what's immediately visible.