(Kibum Park/Raad, LLC)

Imagine an inviting green park with tall, shady trees and wide swaths of grassy lawn where you can hear live music or see theater or simply sit quietly soaking up the noonday sun.

Now, imagine that all underground in an old disused parking garage … but still with trees and grass in the bright sunlight - a little less bright, of course, on cloudy days.

This paradoxical vision is already halfway to becoming a reality in downtown Manhattan, a dream made possible partly by fiber-optic technology that can capture sunlight on high rooftops and literally pipe it down to shine further from big underground "skylights."

Dan Barasch and James Ramsey envisioned it all in 2008 when they teamed up with an idea to transform an abandoned trolley terminal, a 1.5-acre lot underneath the Williamsburg Bridge and next to the Delancey St. subway station.

They dubbed their underground park the "Lowline," a nod to Manhattan's popular Highline Park that transformed another swatch of urban blight - in that case an unused and overgrown elevated rail bed.

PHOTOS: Lowline Park Project

Since they teamed up, Ramsey, an architect and principal at RAAD Studio, and Barasch, formerly VP of strategic partnerships for PopTech, have raised more than $500,000 for the project, including a Kickstarter campaign that totaled $155,000.

This past September, Ramsey and Barasch also staged an exhibit at a warehouse on Essex Street, just above where the proposed park would exist, in an effort to show the public what the Lowline could look like.

But lighting the underground space is a challenge and that is where Ramsey's background in engineering comes in; the former NASA employee turned architect had already been working on a way to collect and funnel light when he approached Barasch about the idea of an underground park.

Ramsey and Barasch explain their concept and in more detail here:

The technology consists of fiber optic cables attached to devices Ramsey refers to as remote skylights. Equipped with GPS, these solar collectors follow and capture the sun funneling it down through the cables. The glass surface of the skylights filters out infrared and UVA rays, but still harvests the light necessary for photosynthesis to take place.

For the exhibit, Ramsey and Barasch, alongside a team of volunteers put this technology to the test; together with their team they hand fit together 600 pieces of anodized-aluminum sheets to create a curved dome, a silver canopy that cast the light down on the warehouse space. On the warehouse roof, 20 feet above, six tracking systems collected the light and piped it down to the space below.

"We looked to the way that they build space telescopes to actually cobble together a mesh of flat pieces to create a very completed curved surface, and that curved surface is calibrated to actually deploy the light," said Ramsey, who worked with infrared spectrometry while at NASA.

With the help of volunteers, including engineers and team members from RAAD Studio, the duo created a mock-up complete with moss-covered knolls and Japanese maples. For their installation, they partnered with Sun Central, a Canadian-based solar technology firm, and Arup, a design and engineering firm that is also working on the Second Avenue subway line in Manhattan.

"All of a sudden you have this idea beginning to emerge where you can take this ancient disused space underneath the city and actually turn it into a public space, a garden really, for everyone to enjoy," Ramsey said.

Both Barasch and Ramsey point out despite their success so far, they still have a long way to go before making the Lowline a reality; first, they need to convince city and MTA officials (and ultimately the state) to let them use the site, a process that Barasch says requires both political and public support.

Barasch, who resigned from his position at PopTech in March, is devoting his efforts full time to the project focusing on fundraising and engaging with members of the community.

"This is not a short-term project," Barasch said. "It's very big in terms of its integration with the overall ecosystem of the space, the neighborhood, the subway line, the community and the city and we want to do this right."

If they gain control of the terminal, Ramsey and Barasch estimate the project would cost $50 million in capital costs for construction and may take five to eight years to complete. Nevertheless, both remain determined to see the Lowline complete.

"It taps into this thing that every human actually just needs, which is public space and some semblance of being outdoors as well as being inspired by making the city more beautiful, more livable," Barasch said.

For now, the trolley terminal remains an empty, shadowy cavern with an undetermined future, but one in which Ramsey and Barasch hope they can play a part.