Pelucidar, Skartaris, or Hollow World...it matters not to me. I am fascinated by the idea of worlds hidden inside other worlds. Like Atlantis, they carry a special mystery. Impossible realms filled with ancient wonders that is what they are. Between Hollow Earths, Time Travel, and Atlantis, I think I have most everything covered for crackpot crazy.
Here's a map of Skartaris; it says it all for me: