TerraDave
5ever, or until 2024
In principle, I love Undermountain, and want to introduce my groups to it in the future.
In practice, I have concerns that mega-dungeons currently will get very tiring quickly, and my groups won't have patience for them. I think this one solves the problem by having different environments via gates and teleports to other areas.
That's an old solution. I am guessing for this one the PCs can't go to Mars, or King Kong's Island, or down the Rabbit Hole.