Aaron over at A Paladin in the Citadel recently made this post about the classic AD&D module D1 - Descent into the Depths of the Earth. I had recently been skimming through the module recently, looking for more old school inspiration. I often like to revisit modules that I never thought much of, and this was one of them. I had the D1-2 version which combined the original with Shrine of the Kua Toa, and I also owned the Vault of the Drow, so I had the whole series. They seemed cool, but as a kid they were difficult to use because they were high level adventures that required the GM to do a lot work, and for the players to use something other than brute force. Also, they lacked a defining story or goal (remember my group attacked the keep in Keep on the Borderlands, so you really had to point them in particular direction). I never used them in any of my games.
Today I find them much more interesting. I think they would be better presented as a sourcebook than as adventure modules, but that's a small point. Rereading all 3 modules as a whole gives me a much better overlook of the underworld. Aaron sees D1 as a "megadungeon template", but I see it as even more. The underworld of D1-3 combines dungeon and wilderness settings, and it forces the PCs to make difficult decisions if they try to resupply without returning to the surface. The possibilities here really do seem endless. The one change I would make is to lower the average power level of the inhabitants so that players could start adventuring in this environment starting around level 5 rather than 10+.
In a way these modules remind me of the feeling I had while reading Jules Verne's "Journey to the Center of the Earth". It's really too bad that those movies stink.