1. beneath the surface of the earth
2. secret or hidden from authorities

1. beneath the surface of the earth
2. in secrecy or hiding

1. a subway system
Usage: British
2. a secret organization working against those in power

Examples

• The underground pipes carry water throughout the city.
• During the war, an underground resistance movement helped refugees escape.
• The miners worked underground for eight hours each day.
• The criminals operated underground to avoid detection.
• She took the underground to get to work in London.
• The underground railroad helped enslaved people reach freedom.
• Underground caves provide shelter for many animals.
• The band started in the underground music scene.