The first bullet surfaced just after lunch.

As Jon Tucker sifted soil through a screen in September, a corroded lead slug jiggled into view amid the sand and ash excavated from a pit just a few feet from a fenced-off sidewalk and rushing traffic. Tucker waved to his supervisor, archaeologist Taft Kiser, and held up the bullet for him to see.

Hundreds of artifacts followed, along with the contours of a buried cellar holding a rich trove of Civil War history sealed since a ferocious 1862 battle in this Virginia city, which today lies just beyond the suburbs of Washington.

The discovery amid construction of a courthouse was unexpected. But the site has astonished historians and archaeologists for another reason: It represents a "time capsule," in the words of Kiser, undisturbed through more than a century of urban construction around it.

Since then, the crew's shovels and trowels have scraped away cinders and sand to reveal the basement's contents: Dozens of bullets. Buttons from Union jackets. Shards from whiskey bottles. A metal plate from a cartridge box. Chinstrap buckles. Tobacco pipes. A brick fireplace and charred floorboards.

"This is a once-in-a-lifetime chance," said Kiser. With the project paused, the team raced to document what they concluded was the basement of a building set afire shortly after the Battle of Fredericksburg. The timing was opportune because the battle's 150-year anniversary is next month, and Fredericksburg has been preparing to mark the sesquicentennial.

"No one, of course, calculated that, but it is a pleasant happenstance that most of us in our lifetimes won't ever see again," said John Hennessy, a National Park Service historian.

Such sites are a kind of archaeological Brigadoon, the fictional Scottish village that appeared once a century.

In Fredericksburg's case, city planners hired Kiser's firm to investigate the historical significance of the property chosen for the $35 million courthouse complex, as they would require a private developer to do in a historic district.

There was no expectation that the investigators would find anything. When the archaeologists initially checked city records, they were unable to find any indication that a building had been on the property before 1886.

"We're ecstatic about what we found," said Robert K. Antozzi, city coordinator for the courthouse project. "Now we have a major expansion of the story of Fredericksburg, and that's really exciting."

Shortly into the dig, the crew discovered a sandstone cellar wall -- a clue that something was preserved below. When the crew dug at another location, it found a brick wall flush against the sidewalk. The dig revved from a sleepy investigation into a dash to extract as much information as possible.

The crew also discovered an 18th-century well and latrines across the site, where livery stables once stood.

Near-perfect preservation

The near-perfect preservation of the site has helped to paint a vivid portrait of the aftermath of the battle, when Union Maj. Gen. Ambrose Burnside tried to take the city from Confederate Gen. Robert E. Lee, as Union troops forged toward Richmond.

When Union forces charged on Dec. 13, 1862, Lee's men were perched on the heights above the city. They easily repelled the Union soldiers, inflicting terrible casualties. Afterward, Union soldiers likely sheltered anywhere they could, including in the basement Kiser's crew discovered.