"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.

Most, but not all.

"If you're walking with me," Manning says, "you have no choice but to know what happened here."

THE FORGOTTEN CRASH: History lost and relived

On a fog-shrouded evening on the penultimate day of 1906, a dead-heading train roared down this stretch of tracks near Washington's Catholic University, coming upon a slower passenger train heading the same direction on the same track. There was no time to stop.

Railroad workers have an antiseptic -- but descriptive -- word for what happened next: Telescoping.

The massive steel engine of the speeding train plowed through the flimsy wooden passenger car of the slower train, killing and dismembering its occupants. It plowed through the next car as well, and the one after that. When the trains came to a stop, cinders and soot from the locomotive's fire box rained down on the splintered wooden planks, clothing, Christmas gifts and human remains. Fifty-three people died, and more than 70 were injured.

Today, the "Terra Cotta" crash is all but lost to history. Every day, thousands pass the site, where there isn't even a hint of the horror that happened.

But Richard Schaffer, a D.C. firefighter who spent 10 years researching the crash, says Terra Cotta nonetheless changed railroading. It hastened the conversion of passenger cars from wood to steel and led to improvements in railroad signaling. That happened, he says, because the crash happened on "the route to Congress."

There's a saying, sometimes attributed to Mark Twain: "History doesn't repeat itself, but it does rhyme."

History rhymed in June 2009 -- nearly 103 years later -- when a D.C. Metro subway train plowed into another subway train. The cars telescoped, killing nine and injuring dozens.

"The irony was it was practically the same location and practically all the same problems, human error, signaling problems, construction quality of the trains," Schaffer said.

Both wrecks deserve to be remembered.

"If you forget what's happened before you," Schaffer says, "you don't have a foundation to live upon."

CONGRESSIONAL CEMETERY: The last hurrah

Can there be any doubt what happens here when the sun goes down?

Can there be any doubt that, when the gates close and the last visitor leaves this historic burial ground, band leader John Philip Sousa reaches for his baton, Civil War photographer Mathew Brady tweaks his camera, and J. Edgar Hoover tries to keep the whole mess under control?

This is Congressional Cemetery, where Washington's political and social establishment rests in eternal peace. In the 1800s, its heyday, this was the site of grand funeral processions. Tens of thousands of Washingtonians would gather to watch soldiers carry fallen leaders down a slate path to graves or crypts.

"I'm sure there are quite a few secrets buried here," says Abby Johnson.

Abby and her husband Ronald, professors of literature and history respectively at Georgetown University, take me to the "Public Vault," a crypt the size of a one-car garage. Built in the 1830s, the vault was used to store the bodies of public officials until the ground thawed, or until they were moved to other locations.

You need a skeleton key, of course, to get inside.