By day they work as computer programmers and stock boys and academics. But at night they are known as urban explorers. The Brooklyn Bridge, London’s Shard, Notre Dame—each structure is an expedition waiting to happen. Each sewer, each scaffold, each off-limits site is a puzzle to solve. No wonder the cops are after them. Matthew Power embeds with the space invaders and sees a world—above- and belowground—that the rest of us never knew existed.