Colin Marshall talks with Geoff Nicholson, author of nonfiction books like Day Trips to the Desert, The Lost Art of Walking, and Walking in Ruins as well as such novels as Bleeding London, Gravity's Volkswagen, and The Hollywood Dodo. His latest is the novel The City Under the Skin, a story that combines Nicholson's familiar urban exploration tropes with murder, kidnapping, and forcible tattooing.