- located or occurring below the surface of the earth
- underground parking garages
- an underground explosion
- an underground deal
- The drugs are supplied through an underground network.
- She loves the city's underground music scene.
- underground newspapers/movies
- The wires run underground.
- animals that live underground
- They had been living underground as fugitives.
- The political party went underground [=started working in secret] after the new government took power.
- I've ridden on the New York subway, the Paris Metro, and the London Underground.
- She commutes by underground.