Hollow Earth is an old theory that is parallel to the flat Earth conspiracies. Theorists part of this community believe that the Earth is actually hollow, but what exactly lies within it varies. The list of beliefs goes as follows: an inner sun, an inner surface with land and sky, advanced or ancient civilizations, prehistoric or giant creatures, large caverns, subterranean oceans and rivers, tunnel networks, secret bases, non-human creatures, obscure libraries or spiritual centers, and a world of altered physics. There is not exactly tangible proof that anything like this resides under the Earth, but the appeal to something like this is that there is not a concrete way to prove any of these theories wrong since we cannot delve into the depths of the planet ourselves.
Link to first video