I'm watching Mimic right now. Somehow it isn't 100% successful, but it's hard to say why not. It does a lot of things right, I think.
Anyway, what are some other horror movies (or books!) that have a lot of scenes, or at least one really important scene, somewhere underground like a sewer or a subway? I'll start.
It
Relic
Reliquary (book)
CHUD
Mimic
Return of the Living Dead 2 (just a couple quick scenes in that huge sewer pipe, but they are important scenes for the movie)
Midnight Meat Train
This horror short film:
Resident Evil (arguably)
Demons (1985) (the beginning)
American Werewolf in London
Cloverfield
and I see some more when I start Googling, but nothing I've seen personally. Anyone got any good ones to mention?