Five years ago, the economy took a dive and architect James Ramsey went underground. Ramsey is the founder of RAAD Studio, an architecture firm located on Manhattan’s Lower East Side. When the financial markets stalled, so did many of his projects. “I suddenly found myself with plenty of free time, and so I started going urban exploring,” Ramsey says.

Ramsey knew someone at the New York City Metropolitan Transportation Authority (MTA) who was a subway history buff and that’s how, one day in 2008, he found himself 25 feet below Delancey Street, one of New York’s busiest thoroughfares, standing inside what was once the Williamsburg Trolley Terminal. Originally opened in 1908 as a depot for streetcars, it was decommissioned in 1948 when the city stopped using the trolleys. The 60,000-square-foot space, owned by the MTA, had been vacant for decades, but the remaining architecture contained the trademarks of solid 20th-century design—muscular steel rivets, cobblestone floors, vaulted ceilings. “It is a stunning, cavernous space,” Ramsey says. “Hundreds of thousands of people traverse overhead not knowing what’s underground.”

Ramsey couldn’t shake the idea that there were several city blocks of unused acreage smack in a congested neighborhood desperate not only for public space, but green space. So what if it was underground? Maybe, he thought, you could channel light like a liquid and irrigate the space with enough sunlight to support photosynthesis. Maybe, he thought, you could build New York’s first subterranean park.

Ramsey began tinkering with a design for a contraption that could collect sunlight above ground, bring it through the street using fiber optic cables, and then redistribute it into the subterranean space using a reflective dome. He partnered with Ed Jacobs, an old friend and industrial designer, to build proof-of-concept models, and then displayed the small prototypes in the window of his architecture studio. “They actually worked,” Ramsey says.

Ramsey and a few other designers from RAAD Studio created computer models showcasing the potential underground park design, replete with trees and greenery, walking paths, and undulating expanses for recreation or relaxation. Ramsey dubbed it the Lowline, a nod to the High Line’s success in transforming disused urban infrastructure on Manhattan’s West Side into a vibrant public amenity. Soon, the concept of the Lowline caught on. “We had these startling designs and the more I showed them to friends, the more encouragement we got to put it forward to the politicians and the community,” Ramsey says. Ramsey, joined by community outreach expert Dan Barasch, formed the Lowline nonprofit organization in 2011 to advance the idea.

But was the daylighting concept truly feasible? To find out, Ramsey partnered with architecture and engineering firm Arup. Star Davis, a lighting designer with Arup’s New York office, conducted an extensive daylighting study surrounding the project. “This isn’t like any space that currently exists and there’s no way that traditional daylighting techniques could be used,” Davis says. For one thing, heavy automobile traffic above meant that a large aperture opening to the sky was not possible.

But Davis and Ramsey knew that daylight—not electric light—was the goal. “Natural sunlight has a magnetic pull for people,” Davis says. Both favor the quality of crepuscular light, the way sun streams through a tree canopy or a cloud cover at sunset to create resplendent shafts of light. “There is never any confusion that that is daylight,” Davis says.

The quality of light diffused inside the Lowline would need to have enough illuminance to support photosynthesis, but not so much that it would blind visitors. “You don’t want to create a glare bomb,” Davis says.

So Davis began a lengthy study of the luminance ratios of daylight at different hours in the city. She concluded that the “magic hour”—that moment when the sun begins to set and ignites the sky—was the light level that they should aim for. Davis also evaluated the solar exposure at the site using a sun-path diagram, which illustrates the path of the sun over the course of the year and throughout the day. Overshadowing diagrams show the shading effect of neighboring buildings.

Based on the conditions above ground and the desired quality of light below ground, it was determined that a heliostat would be the best solution to harvest direct daylight. A heliostat is a solar tracking mirror that redirects light to a fixed position; it can be fastened above ground to a neighboring building or structure. A computer program that tracks the sun moves the mirror to capture the sun’s light throughout the day. The light can then travel to the Lowline using either reflective tubes or fiber optic cables designed specifically for direct solar capture. Davis and Ramsey are still exploring the best options for tunneling the light underground, with the goal of finding a tube or cable that protects the spectral transmission of the light so that it retains its color temperature. Once underground, the light will be diffused via reflective panels designed by Ramsey with the help of engineers.

Heliostats are not new to New York. In Lower Manhattan, Battery Park City’s Teardrop Park South successfully uses heliostats to brighten shaded spots. But redirecting enough light to a space such as the Lowline is a unique application of this technology. The Lowline and Arup reached out to manufacturers who are pioneering the collection of direct sunlight, such as Sun Central, which makes the SunBeamer, a mechanized solar-tracking device capable of being customized for architectural uses.

Last spring, Ramsey believed it was time to test the lighting design’s efficacy, as well as the public’s interest in an underground park by building a sample of the Lowline above ground. “A very next natural step was to put on an art installation about the technology,” Ramsey says.

The Lowline nonprofit started a campaign on Kickstarter and, in a matter of days, raised more than $155,000 to build a full-scale mock-up. The exhibit opened in September 2012 in a warehouse on Essex Street, not far from the Lowline site. After blocking out all light, Ramsey and his team used more than 600 anodized-aluminum panels to craft a complex compound curve on the ceiling to serve as a solar canopy. “We found inspiration in space telescope design,” he says.

Six heliostatic devices were clustered into an existing skylight. Then, Ramsey says, “we plugged in the sun.” The sunlight filtered onto a mossy knoll where a Japanese maple tree and ferns thrived. “We even sprouted [edible] mushrooms,” Ramsey says.

Over the course of the two-week-long exhibit, 11,000 individuals stopped by. Some people rolled out blankets and picnicked, others read books, and suddenly a dark warehouse became a social gathering spot. The press also poured in, and the Lowline’s story was picked up in magazines and newspapers around the country. Today, the momentum for the project continues as the nonprofit works with city officials to acquire the space. Ramsey is confident that the MTA will move forward and work with them on developing the site.

While the Lowline’s principal focus is about bringing light below grade, the equipment needed to do so lies, in part, above grade. To that end, Ramsey and Davis envision that the heliostat skylights would be incorporated into urban furniture that would also double as infrastructure—objects such as sidewalk benches or bike storage. It would also announce the existence of the Lowline to people who are walking or driving by. In many ways, the Lowline challenges not only the traditional understanding of what a daylighting strategy can be, but the extent to which light can be the primary driver in transformational design.

“The Lowline is an opportunity to use light that effects change on a vast urban scale,” Ramsey says, which is not a bad outcome from a slow day at the office.

Project: The Lowline, New York (Under Delancey Street between Essex and Clinton Streets) •  Architect: RAAD, New York •  Industrial Design Consultant: Design Necessities Initiative, Brooklyn, N.Y. •  Lighting Design Consultant: Arup, New York •  Structural, Mechanical, and Electrical Engineering Consultant: Arup, New York •  Real Estate and Economic Development Consultant: HR&A, New York •  Proposed Project Size: 40,000 square feet •  Projected Project Costs: Confidential