As I was reading a new series of books a while ago (The Seventh Tower), i notice it is quite similar to planeshift in a few ways...
1. Both of them take place in an underground land, where no sunlight can reach.
2. They both use a great crystal to light the land, and smaller crystals can be used to light rooms, buildings, etc.
(in the seventh tower, they are called sunstones)
3. Planeshift has 7 levels in its stelactite, and the seventh tower's land has 7 levels to it also
Is this a coincidence? I realize that this series came out before the game did.
If you were wondering to see if I was serious, or if you just want an interesting read, here it is
http://www.scholastic.com/titles/seventhtower/index2.htm