The Mole People is a 1993 book by Jennifer Toth telling the story of "the mole people", homeless people living in tunnels underneath Manhattan. This books is mostly anecdotal in character, and tends to focus on a few personalities, rather than try to give solid sociological data on the underground population, which is probably just as well, since such data probably doesn't exist.
Ms. Toth, to her credit, is quite skeptical about the rumors she hears about the underground, but even discounting many of the stories, she has had eye witness proof that there are indeed many people living in various tunnels underground, a number she estimates as perhaps as many as 5,000. These people live in many different circumstances, from the alcoholic who passes out for the night in a close to the surface subway tunnel to highly organized communities of up to 200 members which prohibit drug use and have elected officials, that live a hundred or more feet underground.
If I gained anything from this book, other than a sick voyuristic peek into a lifestyle that is hard to imagine, it is the many different ways people can respond to the strangest circumstances. Many of the people living in the tunnels are drug addicts, and many of them have problems with mental illness. However, despite this, and despite the violence of their lives, these people try to organize into community to make the best out of what they have and to preserve what is important to them. Some of these efforts come to nothing, while some seem to be quite succesful.
While this book does provide an interesting introduction to a subject that does not have a lot of material written on it, it does not provide a lot of insight into the causes of homelessness, and why people migrate underground, and whether that migration is different than being homeless above ground. Since Ms. Toth's means of exposition is page or two page long descriptions of different tunnel dwellers life stories, a really complete understanding of what it means to live so cut off from "normal life" really can't be reached.