There is, says Raleigh News & Observer staffer Toth, a city below New York City: a fantastic underworld of men, women, and children who are born, live, and die in the darkness beneath the streets. In the early 90's, the author, then a Los Angeles Times intern, spent a year exploring that nether world, preparing this startling report. Toth first heard about ``the mole people'' from a child who claimed that her classmate lived underground; further research brought the author into contact with Sgt. Bryan Henry, a Grand Central Station cop who introduced her to one ``J.C.'' (most of Toth's homeless use pseudonyms), the ``self-described'' spokesman for an underground community of 200--a large but not unprecedented number for one of the dozens of camps, gangs, and roving bands that Toth found in the tunnels. These tunnels--including gas and sewer lines as well as abandoned subway tunnels and stations--honeycomb the city's foundation, descending to seven levels and housing perhaps 5,000 lost souls. To the uninitiated and, at first, to Toth, the tunnels are terrifying: She walks them both guided and alone, aware of forms flitting past, of rats and madmen. She visits camps whose members stay below for weeks at a time; she watches a ``filthy and bearded'' loner skewer and roast a ``track rabbit''--a rat; she talks to graffiti artists, women, teenagers, and a kill- for-hire gang whose services cost $20. Pausing in her chronicle, she surveys underground life in history and literature, from Egyptian slaves living in mines to Ralph Ellison's Invisible Man. Finally, Toth flees the city's depths, her life threatened by a mole man who thinks her a police informer. The life expectancy of the average mole person, stricken by drugs and disease, is under five years. Toth's unusual sociological adventure story, then, is as saddening as it is gripping.