Subterranean means existing or happening underground, beneath the earth's surface. When explorers discover a subterranean cave system, they're finding passages and chambers hidden below ground. Subway trains run through subterranean tunnels carved beneath city streets.

Many creatures live in subterranean environments: moles dig subterranean burrows, while blind cave fish swim in subterranean pools that never see sunlight. Some cities have subterranean infrastructure like water pipes, electrical cables, and sewage systems running below the pavement.

During World War II, people in London sheltered in subterranean stations during bombing raids. Ancient civilizations built subterranean tombs and storage chambers. Today, some modern buildings include subterranean parking garages or basements.

The word can also describe something hidden or secret, operating beneath the surface of normal life. A spy might work for a subterranean organization. But subterranean simply means underground: below your feet, beneath the soil and rock, in the hidden world under the earth.