Behind the doorway of a hulking, unassuming warehouse on Essex Street in Manhattan's Lower East Side, a fascinating—and potentially game-changing—experiment is taking place. This is where you'll find the Lowline Lab, a prototype for an New York City's first underground park, which until now has existed only as an idea. The lab, which opened to the public on Saturday, is the culmination of years of work by its creators, Dan Barasch and James Ramsey, who have been tinkering with plans for the space since at least 2008. The idea is to create a green space deep below NYC's streets by projecting the sun's rays down into a disused trolley station, a complicated process that the Lab is now putting on full display. "We're kind of blown away now that it actually exists," says Ramsey. "To our great delight, all of our equipment and technical pieces are performing in excess of our calculations."