The penitentiary is now closed and largely demolished. The land is part of Fort McNair at the southernmost point of Washington.

Visitors -- mostly lawyers and Civil War buffs -- are frequently overwhelmed when they enter the room, said Susan Lemke, a special collections librarian who has accumulated artifacts related to the trial. "There's no substitution for actually witnessing or being in the middle of a historic site like that," she said.

THE GALLOWS: Where generals "serve," conspirators hanged

Michael Kauffman is struck by the incongruity of it all.

On the edge of a Fort McNair tennis court, where generals now casually toss their gym bags, Abraham Lincoln's death was avenged.

Here in this spot, near the penitentiary room where the sentences were handed down, on a miserably hot day in July 1865, Union Army Capt. Christian Rath raised his hands and clapped three times. On the third clap, soldiers knocked supports out from under a gallows, and four prisoners fell. Their bodies jerked violently at the ends of their ropes. The prisoner in the dress appeared to die instantly. But one of her three accused accomplices writhed for five minutes before surrendering his ghost.

"I am one of those people who think that if you really want to understand history, you have to go to where it happened," says Kauffman, an expert on the Lincoln assassination.

So Kauffman leads me to this empty tennis court. It is drizzly and cold, and there is little here to evince the images and emotions of that hot July day. The penitentiary's tall wall has been demolished, and a building prominent in photos of the hanging has been altered almost beyond recognition.

Almost.

Kauffman shows me the place where the wall met the building. And in my mind's eye, the gallows fall into place.

"There's this strange sort of excitement that you get when you've read about something, and you visualize it, and you think you know all about it. And then all of a sudden you go there and it's right in front of you. It surrounds you. And it's always somehow different from what you had imagined," Kauffman said.

Different, to be sure. But more real than ever.

CHADWICKS: Where the U.S. was shaken, and stirred

It is known as "The Big Dump."

On June 16, 1985, CIA officer Aldrich Ames walked into Chadwicks, a Georgetown pub, with two shopping bags full of classified information and, over lunch, gave them to a Soviet diplomat.

"In those bags was every piece of paper he could get his hands on that revealed almost all of our operations in the Soviet Union," said Peter Earnest, a former CIA official who is now executive director of the International Spy Museum in D.C.

Five to seven pounds of secrets.

The enormity of the breach became known only after the Soviet Union began rounding up some of the United States' most valuable assets in Russia. At least 10 were executed.

The CIA launched a hunt for a possible mole. It compiled a list of 190 CIA officers with access to relevant classified information, and culled it to 28. And in 1994 -- nine years after the Big Dump -- Ames and his wife were arrested.

Earnest says he doesn't "romanticize" the Chadwick's site, but says "the repercussions of what he did ripple through the government today -- the need to have more polygraphs, the concerns about our records ... the nature of the questions asked."

It's also a waypoint in the Spy Museum's bus tour, which notes the role that Ames' "high-maintenance" wife Rosario played in his betrayal of his country.

Tour guides note that after Ames was arrested, FBI agents who eavesdropped on their conversations made an astonishing comment: They were so disgusted with Rosario's constant badgering about money, her criticisms of Ames and her treatment of their son that although they could never forgive Ames for spying, they said, they would have understood if he had killed his wife.

ALEXANDRIA SLAVE PEN: From slave to freeman

"PRICE, BIRCH & CO," the sign read. "DEALERS IN SLAVES."

The sign is long gone, but the building, known as the "Alexandria Slave Pen," still stands in Alexandria, Virginia, just across the river from Washington.

"I often tell my students, 'You've gone into towns where you just see row after row of car dealerships. Duke Street was that -- but slave dealerships,'" says Chandra Manning, associate professor of history at Georgetown University.

In 1861, the slave trade was thriving when Virginia seceded from the union. But on May 24 of that year, the Union Army's First Michigan Infantry marched into town, and one of the first things it did was liberate the slaves.

Ironically, the slave pen became a refuge for runaway and freed slaves seeking the protection of the Union Army.

Today, 1315 Duke Street is home to the Alexandria branch of the National Urban League, a civil rights organization. A historical marker stands outside, and there's a small museum in the basement.

But Manning believes most passersby have no idea about the building's horrific past.