The title of this documentary by first-time filmmaker Marc Singer is no metaphor: homeless squatters, tired of the hassles of the street, go underground to set up a hardscrabble community in a sunless Amtrak tunnel deep below Manhattan. It looks kind of fun at first, in a kids'-treehouse Robinson Crusoe kind of way, and some of the subjects, like the addled guy going through Polaroids of his deceased pets, have a goofy cartoon charm. The lightheartedness dissipates, however, when the crack pipes and rats make the scene and the tunnel dwellers tell their tales to Singer's black-and-white camera. Parental neglect and abuse is a common theme; some are victims of it, others weep with guilty memories of having inflicted it.

Like the rag-tag shelters, Singer's film is a bit of a patchwork with not much to hold it together. Although it captures the sometimes fascinating, sometimes revolting details of it subjects' daily lives, it doesn't delve very deep into why they ended up in society's lowest circle, or ask what, if anything, can be done about it. The ending is happy but perfunctory and pointless -- this exploration of the dark doesn't shed much light.