On September 18, 1793, President George Washington climbed into a trench, set a silver plate on the ground and told laborers to lower the cornerstone of the Capitol of the United States.

On the Capitol lawn Saturday this historic moment will be re-enacted.

However, there is a problem: Nobody is sure where the stone is.

In a city where history reigns, it's hard to believe leaders have lost the Capitol cornerstone. On the other hand, in a city where government has often been charged with wrongheadedness and confusion, maybe it isn't so surprising.

There is a 1873 bronze plaque embedded on a wall in the lower level of the Capitol that deceptively boasts: "Beneath this tablet the cornerstone of the United States of America was laid by George Washington."

For decades people were content to trust in American forebears and believe that the cornerstone rested below this plaque.

"Everybody has always just assumed that that's where it was and that's where we were looking,"said George White, the Capitol architect.

But excavations in the late 1950s, when the Capitol's foundations were underpinned for new construction, proved this claim false. After exposing the building's foundations, a metal detector was used to search for the silver plate buried with the cornerstone, but neither the silver nor the stone was found in the purported resting place.

Three years ago, White began to hunt for the cornerstone after an engineer became possessed by the mystery.

"He just became interested in looking for it and and started, on his own time, just scratching around thinking maybe he'd find it," White said. "When I learned about that I said, `Well that's not a bad idea, but we ought to do something more organized.' "

In the fall of 1991, White asked members of the U.S. Geological Survey to use the latest and least invasive technology to search for the stone a second time. The survey team tested the soil for traces of silver that might have leached from the plate into the ground over the two centuries, but still there was no hint of a cornerstone in the area beneath the plaque.

To unravel the mystery, the Capitol staff turned to the archives. From the minutes of an 18th Century meeting, William Allen, the Capitol's architectural historian, realized that people had been looking for the cornerstone in the wrong place.

When the original Capitol was first constructed, before being burned by the British in 1814, only the House and the Senate chambers existed and an outdoor walkway connected the two buildings where today's central dome stands. Because the Senate chamber was built first, historians believed that the cornerstone was laid in the southeast corner of this building.

However, Allen discovered that the foundation for both chambers was laid at the same time, meaning that the southeast corner was not under the Senate chamber, but more likely under the House chamber to the south of the Senate.

Again, the U.S. Geological Survey was called in, but this time it excavated under the House chamber. In a 3-foot deep, unmarked pit near the House coffee shop, underneath the bustling tourists a floor above, White believes the team may have uncovered the missing stone.

"It looks like it was prepared for a ceremony," White said. "It's a big flat chunk of stone that weighs several tons by our estimates, from its size and composition, and it's not something you'd be casual about."

But Allen is not as convinced as White because crucial evidence is lacking, he said. He is intent on finding remains of the silver plate despite theories that it may have been stolen or accidentally discarded. Although silver was detected in soil samples, it is not more than is usually present, he said.

"Without that silver plate, who's to say?" Allen asserted. "In terms of location, it's right. In terms of size, it's right. Every critieria are there, except for the silver plate, which unfortunately is the most important criterion."