Dungeon Planet

When I first started learning about Dungeons and Dragons, one of the things I found interesting is that most of the action took place in environments below the surface. I could imagine people playing in hundreds for different caverns and caves. With that amount of locations it could fill a small world. So I began thinking of such a planet where the entire volume is filled with nothing but mazes of rooms. The levels would be layered like an onion. Of course, the levels closest to the center would have less gravity. One of the things I added to this is a tunnel through the center that leads straight from pole to pole. This would be a bottomless pit since it would have no bottom and that after some one falling passes the center they would be pulled back.