1. existing, occurring, or done beneath the earth’s surface; underground
Usage: formal; less common than ‘subterranean’

Examples

• The miners discovered a subterraneous cave system.
• Ancient civilizations built subterraneous storage chambers.
• The archaeologists explored the subterraneous ruins.
• Water flowed through subterraneous channels.
• The building had a subterraneous parking garage.
• Subterraneous tunnels connected the fortress walls.