With the opening of Dupont Underground to the public, images of playpen balls are about to flood Washingtonians’ Instagram and Facebook feeds once again—this time in the form of an Minecraft-meets-Lego art installation that was borne out of an international design competition.

Raise/Raze marries the playfulness of the National Building Museum’s The Beach (which was the source of the exhibit’s raw materials) with the scale and otherworldly imagination of the Renwick’s WONDER. Both drew large lines of gawkers, and the response to Dupont Underground’s first public exhibit has been equally enthusiastic; tickets have already sold out.

Councilmembers Jack Evans and David Grosso were on hand with giant scissors to help cut the “ribbon” (a string of balls) for the first new use of the trolley tunnels underneath Dupont Circle since a food court made a brief appearance in 1995.

“I thought I was coming down into Metro, but this looks better,” said Evans, who seized the chance to advocate for using art to enliven WMATA’s train stations. Grosso added: “This is the kind of art exhibit that’s going to put D.C. on the map for things like this.” He also admitted, “it’s been a long time since I’ve been down here. I won’t say how I got in last time.”

Between 1949 and 1962, the tunnels ferried commuters downtown via streetcar. It was briefly used as a fallout shelter, but mostly sat vacant until developer Geary Simon made a deal to establish an underground food court. Poorly designed and underused, Dupont Down Under opened in 1995 and closed the next year.

In more recent history, the Arts Coalition for the Dupont Underground has been dreaming up new uses for the cavernous space. In 2014, they were awarded a five-year lease to experiment and find out what works in the chilly, poorly lit tunnels. First, though, they had to spend some time tearing out the old food stalls and getting rid of years of detritus.

With clean-up underway, the organizers put out the call for an artwork designed specifically for the new venue, using the more than 650,000 balls that made up the The Beach’s ocean/ball pit last summer.

The New York-based firm Hou de Sousa won the commission with an idea to turn the individual spheres into blocks, which visitors could then reconfigure.

Nancy Hou and Josh de Sousa spent a lot of time thinking about hot glue and velcro—at one point counting the individual spikes on the hook-and-loop fasteners. It was all worth it, Hou says, when they finally figured out a way to make the concept work, and were chosen from a strong field of entrants.

“This is as great as anything I could have imagined when we put out the call,” said Braulio Agnese, who stepped down from the Arts Coalition for the Dupont Underground after leading the group for years. Scanning the artfully placed blocks, he added: “there’s something about those translucent balls that really inspired people.”

That includes the hundreds of volunteers who agreed to head down into the tunnels in their spare time to do the repetitive work of hot gluing balls together.

“Our big fear was the time and labor,” recalls Hou, who was delightfully surprised and thankful to find all the extra hands when she and de Sousa came down on weekends to work on the installation.

Still, they had to significantly scale back the design from their proposal, which included five different zones in the curved space. Only the area at the back end of the space sticks closely to their original plans.

“We had plenty of material,” said Philippa Hughes, who sits on the Dupont Underground’s board and has helped lead the project. In fact, there are hundreds of thousands of balls left over. But time constraints mean that they had to scale down the ambitious project.

Hou estimated that it still took more than 3,000 hours of labor to get to their modified design.

And that design is about to change, literally. Visitors (who shelled out $16.82 for tickets) will be able to reconfigure the blocks according their own whims. But they will have to enjoy themselves in the moment—and wait to post their selfies. There’s no cell service in the tunnels.