More than a decade ago a group of New York City residents launched an ambitious experiment to build a park atop an expanse of abandoned elevated freight train tracks. Today the High Line, which opened in 2009, provides locals, commuters and tourists with more than a kilometer of green space several meters above the urban bustle below. Emboldened by the project's success, a team of designers and engineers has proposed the polar opposite idea: transform a deserted underground trolley depot into a haven for leisurely recreation.

New Yorkers are getting a glimpse this month of what the Lowline park might look like thanks to an exhibit demonstrating technology that channels enough sunlight to subterranean spaces to support plant life. The exhibit—on display September 15–27—features a skylight that delivers the sun's energy from an outdoor solar collector to an indoor canopy for distribution. Living below the aluminum canopy is an impressive array of flora specially chosen for its ability to thrive in low light.

Lowline organizers are pitching the park as a space covering more than 5,500 square meters with a five-meter-high ceiling. The park, which would feature art exhibits and food vendors alongside the subterraneous photosynthesis, would inhabit the former Williamsburg Trolley Terminal, which opened in 1903 as a depot for streetcars ferrying passengers between Brooklyn's Williamsburg neighborhood and Manhattan's Lower East Side. The terminal has been abandoned since the city discontinued trolley service in 1948.

Given the absence of ambient natural light, "remote skylight" technology developed by Lowline co-founder James Ramsey will be critical to the park's success. The remote skylight will use a reflective, parabolic solar collection dish outdoors to gather and concentrate sunlight. This dish will have a tracking mechanism so it can follow the sun across the sky. Fiber-optic cable will transmit captured solar radiation to the park; a series of domelike fixtures will use lenses and reflectors to distribute the light throughout the Lowline.

The fiber-optic cables will allow Lowline organizers to set up as many remote skylight fixtures as they like, says industrial designer Edward Jacobs, who is working with Ramsey's Raad Studios as a consultant on the project.

The exhibit's lighting setup differs somewhat from the envisioned Lowline scheme. Because the exhibit is in an aboveground warehouse, it collects sunlight on the roof and channels the rays directly through a circular array of six tubes into the building—no fiber optics needed. The tubes, each of which is about 53 centimeters in diameter and contains an arrangement of mirrors and lenses, send sunlight down to three hexagonal reflector shields hanging from the center of the circle. These shields bounce the light back up to a 10.5-meter-wide reflective canopy surrounding the tubes, disbursing light on a cluster of vegetation below.

Jacobs sees the Lowline as an extension of city residents' efforts to make use of abandoned space to improve their environment through parks, gardens and other environmentally friendly projects. Eventually one could envision New York's densely populated urban spaces on the street level being sandwiched between green spaces on rooftops and underground, he says.

In terms of the underground greenery, organizers have yet to determine exactly which types of plants will be featured in the park. The exhibit includes a variety of vegetation that thrives in low light, says environmental designer Misty Gonzalez, founder of Hortus Environmental Design, who arranged the plant life on display. This vegetation includes many species that typically live under forest canopies, including nonvascular plants (moss, in particular), Japanese spurge (Pachysandra terminalis), autumn fern (Dryopteris erythrosora), lilyturf  (Liriope muscari), Korean rock fern (Polystichum tsus-simense), pearl oyster mushrooms (Pleurotus ostreatus), along with a Japanese maple tree (Acer palmatum).

Gonzalez acknowledges that an underground garden would bring challenges beyond adequate lighting, including those related to irrigation and possibly pest control. "We're introducing an element that will create its own ecosystem within the Lowline," she says, adding that this ecosystem would likely include insects such as centipedes and millipedes. As with the rest of the Lowline, the details still need to be worked out, including how any fluctuations in lighting—thanks either to varying outdoor conditions or the sunlight delivery equipment—might impact the plants over time.

Ramsey and Lowline co-founder Daniel Barasch still need permission from the city and the New York Metropolitan Transportation Authority—which owns the trolley depot—to actually construct the park, a process they estimate could take between five and eight years based on how long it took the High Line to go from the drawing board to a major attraction. Given the limited number of parks the city has to offer, urbanites no doubt will appreciate any new green space offered, even if it runs alongside a subway line.

View images of the Lowline exhibit and proposed park.