While New Yorkers might think they're familiar with the subway system, there's a subterranean world beyond the cars that few people get to see: the untraveled tunnels and byways of the subway. Here, graffiti artists have been honing their craft for the last four-plus decades, for only the eyes of other artists and transit workers. With their new book Beneath the Streets: The Hidden Relic of New York's Subway System, Matt Litwack and Jurne bring the works and the tradition to light. "Our book serves as a historical document that preserves the last remaining work from this era," Untapped Cities quotes Litwack, "which has all but been eradicated with the New York City MTA's policy on graffiti in the transit system." Get nostalgic for the subways of yore with a few preview photos from the book.