The Inner Earth is a concept proposing that our planet contains massive inhabited subterranean caverns that at some time contained, or still are home to, entire human civilizations. There are allegedly numerous entrances around the world, particularly in the polar regions, that are said to lead deep underground into these mythological Inner Earth Kingdoms.
Voyage to the Inner World https://www.youtube.com/watch?v=eLPedQdHAKc&t=0s
Antarctica and the Odin Departure https://www.youtube.com/watch?v=ZdtHvwV0RV4&t=0s
Invading Antarctica: Secret South Pole Colony https://www.youtube.com/watch?v=eLPedQdHAKc&t=0s