subterranean (adjective) – under the ground (sub means under in Latin)

Examples of how you can use subterranean:

In the Middle Ages there were no subterranean sewers, so people emptied their toilet bowls in the streets.

There are many subterranean passages under the city of Rome, which were often used as hideaways.

Water flows through subterranean channels and finally reaches the surface in this well.