I haven't looked closely at it, but the Second Darkness AP involves Drow and the Darklands, so it should be fairly undergroundy, at least in the later parts.
Some friends and I are looking to start a campaign, and I was just wondering if there are any dungeon crawl heavy campaigns? The Adventure Paths all look to be more outdoorsy than undergroundsy.
Maybe get the .pdf of Dungeons of Golarion. 5 Megadungeons waiting to be crawled but no real story. you could always make one up?