Underground spaces exert a strong pull on the imagination of most people, although for some this morphs into a fascination bordering on the obsessive. American author Will Hunt is one such person, part of a worldwide community of urban explorers who infiltrate into “the city’s obscure layers”. Though this encompasses more than underground spaces, they are a big part of it, and this book is Hunt’s story of how he fell in love with them. It is one of two big books published only five months apart on the subterranean realm, and I previously reviewed Robert Macfarlane’s Underland: A Deep Time Journey. Here I will turn my attention to Underground.