underground

of or about being under the ground

Example: Underground water is drying up.

secret or not well known

Example: The underground music scene is really interesting.

being or moving under the ground

Example: Let's go underground. It's safer down there.

If you go underground, you keep your activities secret.

The underground is a system of trains that runs mostly under the ground.

Example: The London underground is a great way to move about the city.