IGX

http://www.imdb.com/title/tt0462172/

We picked this up on a whim during a Blockbuster free trial. It is surprisingly good, considering the low budget.
It concerns a trio of "urban explorers" (AKA building hackers, AKA trespassers) who attempt a trip into Moscow's underworld in search of abandoned fallout shelters, Stalin's secret private metro, and the like.

One of them is suffering massive guilt from the death of his daughter the year before, and as the trip becomes increasingly dangerous (radiation, Russian troops on mysterious maneuvers, "Jacob's Ladder"-esque underground mutants) he becomes increasingly unhinged.

The ending isn't a huge surprise, but good use is made throughout of the crumbling infrastructure and the explorers' claustrophobia and paranoia.

Shit, that sounds kinda cool.

I did a bit of urban exploration a long time ago.

A few of us explored some of the Durant Motel in Flint, which was a haven for vagrants and crackheads, and there were bits and pieces of the Capitol Theatre that had been boarded off for decades...we came up from some interesting underground hallways in the basement of a bar, and the owner was fucking pissed about it. And finally, there was a massive rainwater system under Flint that went for miles. 4 weeks after we checked it out, some kid was drowned against a grate during a rainstorm and they really make access difficult.

Yeah, the urban exploration stuff is the best part. The rest is a bit derivative of other movies.

It seemed incredibly dangerous to us while watching it, not only in a "getting crushed by crappy old structures" way, but long-term. As they were crawling along some old pipes and shit I said "You know, they're going to be confusing the HELL out of some oncologists in 20 years or so."