subterranean
adj.existing or happening under the surface of the ground. You use this to describe things like tunnels, caves, or hidden rooms.
adj.existing, situated, or operating below the surface of the earth. Often used in technical or literary contexts to describe geological features or hidden infrastructure.
“The city has a complex subterranean train network.”
“The explorers discovered a subterranean river that flowed through the limestone caves for miles.”
“Urban planners are increasingly looking toward subterranean solutions to alleviate surface-level congestion, proposing vast networks of automated delivery tunnels beneath the city's historic foundations.”
Typically used attributively before a noun; can also be used figuratively to describe secret or hidden activities.
From Latin subterrāneus + -an. Compare subterrane and subterraneous.
Etymology adapted from Wiktionary, available under CC BY-SA 4.0.