Reporter Alix Spiegel reports on a growing movement in cities across the country — Urban Exploration. She accompanies three explorers into an unused New York City subway tunnel. These urban explorers seek out the dark, forbidden and difficult to reach corners of the city — defunct drainage systems, "no access" hotel roofs, the occasional city hall — those places least accessible. The explorers describe the places as the frontiers of the urban landscape. The wear dark suits and ties — "urban camouflage" and share their findings and adventures with other urban explorers via the Internet.