What's the opposite of New York City's High Line? The Lowline, naturally. Lowline is a park proposal to repurpose an underground abandoned trolley terminal below Delancey Street on the Lower East Side.
Rendering: www.thelowline.org

James Ramsey of Raad Studio has a skylighting design to illuminate the proposed underground park. Sunlight would pass through a parbolic solar collecting dish with a sun tracking mechanism. The sunlight would be redirect via fiberoptic cables ("helio tubes") to underground domes, which in turn would distribute and direct the sunlight. In September 2012, the Lowline team built a full-scale prototype of the skylight in a Lower East Side abandoned warehouse, an "Imagining the Lowline" exhibit that attracted thousands of visitors at the time.
Rendering: www.thelowline.org

High Line, the defunct elevated railroad tracks transformed into a pedestrian green space for New York City, has been immensely popular.

Now, NYC is talking about the Lowline, a 116-year-old abandoned Williamsburg Bridge Trolley Terminal, a stark one-acre space of rows of I-beams supporting ceilings that is being envisioned as a repurposed underground park lit by solar technology via a sun collector system on the street above.

The trolley ran from 1908 to 1948, but neglected since. The defunct terminal is adjacent to the Essex Street JMZ subway station, and located in one of the least green areas of the city. Over $1 million has been raised for research and design, but such a project is projected to cost $60 million. A Lowline "anti-gala" fundraising event was held Oct. 8, 2014. The Lowline group is looking to private investors, but says the government has agreed to participate in funding. Construction for the Lowline is not imminent, but 2019 is a starting goal date.