That does sound like a cool idea. How to incorporate the theme with the mechanics?
Maybe you have a start screen or start room showing your Stargate, and you know the address of certain gates. You go and find more addresses to open even more gates until finally you open the gate to the location of, you guessed it, The Amulet of Yendor. Protected by some theme appropriate baddy, a Gah-Ool or something.
EDIT: Actually a 'gate' system instead of a straight dive sounds intriguing. The gates each are 'in range' or connect to certain others, but not to all, so your map of levels is not linear or anything else common, but rather a spider web of nodes...I might use that. Thanks for the idea Meow Meow.