In Samuel Fuller's exaggerated pulp melodrama Shock Corridor, a Pulitzer-hungry reporter admits himself to an insane asylum in order to solve a murder but winds up going crazy instead. Even as B-pictures go, that premise seems wildly implausible, but if Fuller were alive today, he would have to cede his Commitment-To-Journalism Award to Marc Singer, director of the amazing documentary Dark Days. For two years, Singer lived among a small community of homeless New Yorkers in an Amtrak railway tunnel under Penn Station, a dank subterranean hell steeped in garbage, rodents, and human refuse. Employing his neighbors as a makeshift crew, Singer's stark black-and-white photography renders their world with the abstract horror of a German expressionist film, yet he's equally skilled at coaxing the grim personal stories of life underground. Among the tunnel's roughly 75 residents, some of whom have survived there for well over a decade, the majority are crack addicts who value their refuge from the law and are probably too addled to complain about the conditions. But many of Singer's subjects make a strong case for the advantages they have over the homeless on the street, including shelter from the elements, free electricity, and jerry-rigged homes complete with TVs, appliances, and other modern amenities. One enterprising young man even seems committed to a normal workweek, scavenging above ground for recycled cans and edible food on the weekdays while leaving Saturdays and Sundays open "to chill." But Dark Days is hardly a brochure for cheap and unheralded real estate: Without natural light, running water, or waste disposal, the city's so-called "mole people" are at the mercy of filthy rats and infectious disease, not to mention the passing trains. Singer's sympathetic yet unsentimental eye doesn't recoil from unpleasant imagesrepeated shots of a woman nursing a crack pipe are a more effective anti-drug statement than a decade's worth of "Just Say No"and he captures some truly harrowing stories. His only misstep is an abrupt ending that seems tacked-on at best and conspicuously naïve at worst, but even that arises from his deep reserves of optimism and respect for the homeless. Powerful and vividly stylized, Dark Days is a one-of-a-kind testament to human survival in the foulest imaginable squalor.