As cities grow, their infrastructure shifts accordingly to meet new needs. Roads, bridges, tunnels, and waterways change directions, expand, or even become obsolete. The same holds true for urban railways. Take, for example, New York City, with its criss-crossing labyrinth of subways that are the city’s lifeline. Tourists, commuters and other citizens rely on the subway system each day, but not every rail line remains in use. Over time, new routes are created and the city must decide what to do with that precious, abandoned real estate. Sometimes these outmoded lines are demolished or simply paved over and turned into roads. Other times, these government-owned strips of land are re-zoned for the private sector. But still another option is to convert the land into public-use parks, like Manhattan’s hugely successful High Line. The High Line transformed an elevated section of the New York Central Railroad spur called the West Side Line into a urban park. The reclaimed space is over a mile long and runs through the Meatpacking District, Chelsea, and the West Side Yard. This aerial greenway is filled with plants, seating areas, and walkways, and was inspired by the similar Promenade Plantée in Paris, another linear park built on obsolete elevated rails. Lush oases sprouting in the midst of a concrete jungle. Both projects have become popular with residents and tourists alike, and have also spurred economic growth and real estate development for their respective metropolises. But what about all of the tunnels and stations that were abandoned underground? It turns out there are ambitions to convert those into parks as well. Yup, underground parks. “The Lowline" is a proposal for what could be the world’s first underground park, and its name gives a clear nod to its predecessor and sister project in New York. Co-founders James Ramsay and Dan Barasch have designed in detail a plan to re-direct sunlight from the surface down to the darkened subterranean station below. In this way, natural light could be used to illuminate the cavernous space and allow grass, flowers, and even trees to grow where you would least expect. On their web site, Ramsay and Barasch have already targeted the ideal site to build their landmark project. “The proposed location is the one-acre former Williamsburg Bridge Trolley Terminal, just below Delancey Street on the Lower East Side of Manhattan. The site was opened in 1908 for trolley passengers, but has been unused since 1948 when trolley service was discontinued. Despite six decades of neglect, the space still retains some incredible features, like remnant cobblestones, crisscrossing rail tracks and vaulted ceilings. It is also directly adjacent to the existing JMZ subway track at the Essex Street subway stop – so park visitors and subway riders would interact daily.” The ambitious project has already garnered support from politicians on both municipal and state levels, and has also made great strides toward its multimillion-dollar funding goal. Check out their video to see a more detailed vision of the future, as they see it. And this time, it seems like inspiration is flowing the other direction, since Paris might be planning something similar. A recent article reports that “Nathalie Kosciusko-Morizet, a center-right candidate for mayor of Paris and former environment minister, has unveiled a series of plans to turn the legendary ‘ghost stations’ of the Paris Metro into underground oases.” The French proposal goes even beyond parks, opening the possibilities to art galleries, nightclubs, restaurants, and even public swimming pools. These exciting plans all make perfect sense. For cities that are bursting at the seams, taking advantage of underground real estate could be just as important as the instinct to build vertical skyscrapers toward the sky.