If you want to discuss the World of Greyhawk from its creator's point of view, rather than the version developed by TSR after Gary was thrown out of TSR, look at this forum.
If you want to discuss the World of Greyhawk from its creator's point of view, rather than the version developed by TSR after Gary was thrown out of TSR, look at this forum.
For those interested in such a discussion, I believe Mr. Gygax also has a thread on ENWorld where he answers questions.
Additionally, to get back to the FR world before the Jedi Master's scold me. A Greyhawk product called Greyhawk Ruins is a wonderfully detailed dungeon product with wonderful personality. Many of its levels or encounters could be adapted to Undermountain.