I can tell you that our ancestors are obsessed with large caves in a way that suggests that A. They were hiding. And B. There was something down there they wanted.

There are MANY archeological excavations in south america and egypt which are just tunnels (sometimes with stairs) going deep into the earth. We don't know what is at the bottom of them, because we always run out of funding and abandon them.

Human and child trafficking is quite real, and likely involves some underground/clandestine facilities.

He will extend his terrible throne out from middle earth...