Lost City of Gaxmoor is not appropriate, it's above ground, mostly intact and set in a temperate/forested region; its original inhabitants have only just been overrun by evil hordes at the time the adventure starts.
City by the Silt Sea - yeah, that has two or three sunken cities in it, plus an adventure - all detailed in a decent boxed set. Anything that has a 40+ level undead dragon in it as the BBEG gets my vote.
This was the first one that came to my mind, too, and yes--I support Style's suggestion!
If you can find the boxed set, it's not too Dark Sun-ny to make it unusable to a standard campaign (IMO). The Valley of Dust and Fire booklet might not be a bad suppliment to that, either. Both items' geographies are located in roughly the same area of Athas (the Dark Sun world).
City by the Silt Sea popped into my head as well. Aside from a two session stint by portal in my Planescape campaign to deliver a box of unknown contents from a Baernaloth to Dregoth of New Guistinal, the namesake of the box set, I've never run a Dark Sun game. But I've plundered the world for ideas from time to time, it's very cool stuff, and I think it fits the bill for what you're looking for.