Hollow Earth, Inner Earth or Agartha has different ancient species living there which are of course Dwarves/Gnomes, Cyclopses, Tall Greys, Reptilians and alien experiments. There were of course invaders of Hollow Earth such as Pleiadians, Rigel Greys, Zeta Greys and many more. Understand that your Earth was not created Hollowed, it was later designed hollowed by intelligent beings such as Tall Greys, Dwarves and Draconians through different times and Tall Greys and Draconians are still creating hollowed passages and areas. No different than Ants creating Hollowed spaces in their Nests.( shared from extraterrestrials created humans and religions)