WASHINGTON — Roaming the streets of the Dupont Circle neighborhood about 20 years ago, Julian Hunt spotted a grimy staircase leading down from the pavement to a boarded-up door.

He spent many hours on the phone and in the city’s archives, which led Mr. Hunt to crawl through filthy tunnels with a flashlight to discover an old trolley tunnel inhabited by a small group of homeless people.

Since the city’s trolley service shut down in 1962, the 75,000-square-foot labyrinth had been the site of a subterranean murder, rumored ’80s rave parties and a Cold War-era bomb shelter. Now, Mr. Hunt, an architect who was a founder of the Hunt Laudi Studio, has turned the tunnels into the Dupont Underground art space, which draws 3,000 visitors every month.

“It’s one of those spaces in the city that becomes mythical because it just hasn’t been open to the public for so long,” said Brianne Nadeau, a member of the City Council.