On sunny afternoons, Dupont Circle hums with a bustling mix of shoppers, people-watchers and confused drivers. But down a boarded-up staircase, along unseen underground corridors, it is always night.

Flashlight beams fight to puncture the darkness. Debris litters the sun-starved passageways. An empty pack of Merit Ultra Lights sits on a ledge of a gutted laundry. A baby stroller stuffed with rap records teeters on a trash-strewed flight of stairs.

The cafeteria-style food trays with trolley logos scattered in a room are some of the only clues left that hint at the dark area's past -- Dupont Down Under, a short-lived retail complex in an underground structure once used as a streetcar station and fallout shelter. "It's like the archaeology of Washington, D.C.," said Dan Tangherlini, the city's transportation director, who took a tour recently. "Who knew that there was all this space?"

More than sewers, subways and parking garages occupy the city's subterranean world. In the basement of Washington, dozens of feet below the surface, people attend conventions, visit museums, work in air-conditioned serenity, go for a swim at the YMCA, dance in bass-thumping clubs and honor the dead.

They admire the red-bellied piranha residing, of all places, in the depths of the U.S. Department of Commerce headquarters, home of the city's National Aquarium. They squeeze into chambers below the street to operate on some of the city's thousands of miles of power cables, still relying in the Information Age on ropes, ladles and a boiling pot of metal alloy to do their work.

Underground development has flourished in a capital city with security concerns, a height restriction on buildings and a mere 69 square miles. "Part of the reason you're seeing more things underground is . . . that it's really a question of scarcity of space," said Andrew Altman, director of the city's Office of Planning. "Land is at a premium. If you want to be in prime locations, you're looking at going underground or utilizing more of the underground than you would have 10 years ago."

Like the soil itself, Washington's underground is multilayered. There's the traditional office and commercial space occupied by restaurants and corporations. Then there's the more functional guts of the city, a labyrinth of steam tunnels, fiber optics and utility lines. And there's the federal government's rumored passageways that few, if any, have ever seen, such as the Cold War-era eavesdropping tunnel beneath the Russian Embassy in Glover Park. Then there's everything else stuffed into the little-known caverns of a city that often builds down instead of up.

Beneath the Franciscan Monastery in Brookland are the shadowy likenesses of the Roman catacombs to remind visitors where early Christians hid from their oppressors. Below the nave of Washington National Cathedral is a columbarium that is off-limits to the public. In the limestone walls are the cremated remains of 160 people, among them Helen Keller and her teacher, Anne Sullivan.

Under the Lincoln Memorial is a massive, darkened basement where steel-reinforced concrete columns are decorated with construction workers' 90-year-old graffiti -- Mutt and Jeff here, a man smoking a pipe there. Stalactites once formed by the hundreds inside. The National Park Service used to give tours, but no longer. "It was never designed for people's safety," explained the Park Service's Stephen Lorenzetti, chief of resource management for the area that includes the memorial.

Beyond the city's underground chambers, vast amounts of real estate occupy sub-Washington. About 40 percent of the six-block Washington Convention Center -- the city's biggest building -- was placed below grade to blend it into the Shaw neighborhood. The Smithsonian Institution's Arthur M. Sackler Gallery, the National Museum of African Art and the S. Dillon Ripley Center occupy a 360,000-square-foot space that is largely underground. "There wasn't enough room on the Mall for a structure that would house the two museums and the center," explained Smithsonian spokeswoman Linda St. Thomas.

Even more digging is in store. Crews are building an underground visitor center at the Capitol, a three-level facility intended to provide more convenience to tourists and better security for Congress. An underground educational center at the Vietnam Veterans Memorial is in the planning stages. The National Law Enforcement Museum will have the majority of its 90,000 square feet placed below ground when it opens at Judiciary Square in spring 2009.

Subterranean Washington can be a top-secret area, as restricted in these times of heightened security as the airspace above. Officials at one well-known building requested that their underground maintenance tunnels remain unpublicized for security reasons. The 495-space parking garage underneath the National Air and Space Museum has had limited access since 1986, when officials closed it because of fear that terrorists might detonate car bombs there. It is now used for parking for employees and invited guests only, a museum spokeswoman said.

In a historic city where virtually every inch of public space has been mapped, researched and photographed, Washington's underground remains one of the last frontiers of uncharted terrain. No one has a precise accounting of what's below local and federal Washington, or at least no one who is talking. There are vaults no one ever uses anymore, like the men's and women's restrooms below Mount Vernon Square that date to the early 1900s but are now padlocked. And there are the spaces that only workers like Tom Dobson have seen.

"There's only one way out of here, and that's up," said Dobson, 35, as he stood inside one of Washington's roughly 60,000 manholes. The hole, at a Pepco work site at Third and O streets NW, was a few inches bigger in diameter than an extra-large pizza. Dobson, a veteran cable splicer nicknamed "Batman" by co-workers, knows the underground. He has spent countless hours in D.C. manholes, dressed in thick flame-resistant coveralls, fusing cables together with a pot of silvery solder in cramped, hot, high-voltage chambers roughly seven feet deep.

"It's a fun job," he said, seriously.

Below Dupont Circle, the center of one of the city's trendiest commercial and residential areas, is its alter ego: a sunless complex of defunct shops, trolley tunnels and offices that resembles the set of an apocalyptic disaster movie. City officials said it is one of the largest abandoned underground spaces that they know of, a dank, other-worldly realm of streetcars undesired.

One recent morning, Tangherlini strolled through the tunnels -- bravely, a flashlight in one hand and a cup of coffee in the other. Nothing appeared to stir in the tunnels, not even rats, but there were blankets and graffiti, evidence of recent stirrings. "Welcome," someone had written in black marker on a tiled wall, "to the Magic Mushroom Patch."

Tangherlini made his way past the detritus on the concrete floor, past building supplies, a child's bicycle, jackets, old newspapers, lighters. In one room, he stumbled upon some books ("Mastering the Zone: The Next Step in Achieving SuperHealth and Permanent Fat Loss"), and in another he found a surprisingly clean stack of green food trays. "This is a classic example of what you're not supposed to do in a horror movie," joked Tangherlini, as others on the tour wandered into the darkness. A dead body, after all, was discovered here one Sunday evening in January.

Twin streetcar tunnels that opened in 1949 run on both sides of the Connecticut Avenue underpass and skirt the outer rim of the circle. Two station areas total 42,600 square feet, and trolley tubes located 15 to 20 feet below the surface of the road stretch about 700 feet north of the circle and 300 feet south, city officials said. The streetcars stopped running in 1962, and the fate of the tunnels has remained a question mark ever since.

An Alexandria businessman proposed converting them in 1982 into a national sanctuary for the ashes of the dead. That didn't gain much interest. In February 1995, part of the complex reopened as Dupont Down Under, but the food court and retail area closed soon after when the project ran into disrepair, litigation and other problems. The District terminated its lease with developer Geary Simon in 1996, but by then, most of the tenants already had left.

The shops -- Schlotzsky's Deli, Taste of India, Dupont Bagelry -- are nothing more than shells now, but their facades still look like streetcars, complete with plastic wheels. The real trolleys are gone, but along the bottom of the curved, concrete tunnels runs the steel track. Tangherlini said there are no proposals to develop the space.

After the tour, a security officer locked up the staircase, but not before Tangherlini grabbed a few souvenirs of his journey underground to take back to the office: food trays, newly exposed to the light of day.

D.C. Transportation Director Dan Tangherlini leads a tour below Dupont Circle, home of a defunct shopping center called Dupont Down Under. Previously it housed streetcars.Benjamin Johnson, 5, foreground, and his brother, Van, get the attention of a sea turtle at the below-street-level National Aquarium."There's only one way out of here, and that's up," says Tom "Batman" Dobson, working underground to splice Pepco cables. Elsewhere there are steam tunnels and fiber optics.