From TED NYC, 2014.
From the TED YouTube channel:
Dan Barasch and James Ramsey have a crazy plan—to create a park, filled with greenery, underneath New York City. The two are developing the Lowline, an underground greenspace the size of a football field. They’re building it in a trolley terminal abandoned in 1948, using technology that harvests sunlight above-ground and directs it down below. It’s a park that can thrive, even in winter.