I'm aware that such a theory exists, but know only about as much as you posted.
A sea or surface traveler could proceed over an edge of the hole, like an ant crawling over the lip of a coffee mug from the outside to the inside, and not be aware that he was actually entering the interior of the earth.
But you would notice. We see a horizon because we live on the outside of a sphere--a convex surface. On a concave surface like the inside of a sphere, there's no horizon. You'd see the world rising up around you like walls on all sides.
To say nothing of the gravitational weirdness that would occur if most of the mass of the world was over your head.
Several years ago, a group of "believers" informed a member of the Geophysical Institute staff that there was an opening to the center of the earth in the Alaska Range, and that this was an entry and exit point for flying saucers.
Sounds like these people were toking on some pretty chronic ganga...
