The initial reference is Plato's Phaedo from circa 360 BC. He says that Socrates herein explains his belief that the earth is spherical, in middle of the heavens and in perfect equilibrium - going on to explain that there are many vast places in the Earth and apparently a description that appears to be a clear indication of a hollow or very cavernous (likely both) world.
Any recommendations on the best translation? Or do we need to go to Greek to really get to the core of this topic with his writings?
This video may be related... would be curious to hear thoughts (I've not had a chance to watch it yet):
YouTube: Out of Body Experiences in the Phaedo
Sepehr has done a great job on this one. My hat is off to him. It is not one 3 hour video, it is a series of videos, each about ten minutes long, and you watch what you want.