The third book by Elly Griffiths The Chalk Pit, has a section on Underground Societies, people who live in some of our most affluent cities, but they are driven to live below the earth. People who - for whatever reason - aren't welcome on the surface - homeless people, the addicts, HIV-positive. There are subterranean communities all over the world - in catacombs, sewers and abandoned metros. The tunnel people in Las Vegas, the Empire of the Dead in Paris, the Rat Tribe in Beijing. A lot of them are proper societies with electricity and phone lines - even churches and restaurants sometimes. The Rat Tribe in Beijing are mostly migrant workers, some of them brought in to build for the Olympics (The Chalk Pit, p.197).