In New York, where real-estate costs are at a premium, it’s practically criminal to waste space—even the subterranean kind. One underutilized site was a block-long passage below street level that leads from the Columbus Circle subway station to West 57th Street. In 2014, local firm Architecture Outfit began working with Oases Development to devise a plan for the Metropolitan Transit Authority to transform it into a flexible underground food-and-shopping pedestrian street named Turnstyle.