Subterranean realm

Examples:
* A Journey to the Center of the Earth (a story by Jules Verne)
* The Underdark (D&D Forgotten Realms)
* Moria (Middle-earth)

This is a relatively common way to provide players with an alien like environment as there are no such things on Earth occurring naturally or even semi-naturally beyond subways and other tunnels through mountains or other geographical formations. Nothing on the scale this tag is for anyway.

This tag mostly indicates the difference in scope between natural caves and the vastness of things like Moria and the Underdark.