While digging during a project in downtown San Francisco, construction crews were stopped by an unexpected find: the ruins of the city's original City Hall, destroyed by the great earthquake of 1906.

"We were surprised to see it," said historic preservation specialist Rebecca Karberg, according to the Associated Press. "You never really know what's under the surface."

Crews reportedly unearthed the foundation of the building, which was buried beneath rubble in the deadly earthquake.

The San Francisco Chronicle reported:

The imposing old City Hall collapsed in a shower of bricks, stone and steel in the 1906 earthquake. It was the largest municipal building west of Chicago and was so elaborate it took 25 years to build. The City Hall was supposed to be earthquake proof, but it collapsed in seconds after the great quake struck. It had been open for less than 10 years. Its ruins were demolished in 1909.

But while the ruins were demolished, the foundation remained, hidden beneath the soil until earlier this month.

According to the Chronicle, the find is important both to San Franciscans and archaeologists, who are using the opportunity to learn more about construction in earthquake territory.

The demolished City Hall was one of the great losses of the earthquake. In an April 26, 1906 letter to President Roosevelt, Secretary of Labor and Commerce Victor H. Metcalf detailed the destruction:

"As regards industrial and commercial losses, the conditions are appalling," he wrote. "Practically every municipal building is destroyed, forcing the city officials into scanty quarters […] three hundred thousand people were rendered homeless, and their ordinary methods of providing themselves with food, clothing and shelter being entirely destroyed."

San Francisco's new City Hall, the one that stands today, opened nine years later in 1915.

Check out our slideshow of historic moments at San Francisco City Hall over the years:

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  • California Teacher Protests

    Hundreds of teachers protest against budget cuts outside of City Hall. May 3, 2011.

  • Prop. 8 Protests

    Demonstrators protest the passage of Proposition 8, California's ban on same-sex marriage. November 5, 2008.

  • San Francisco Gay Marriage

    Then-mayor Gavin Newsom orders City Hall to issue marriage licenses to same-sex couples. February 14, 2004.

  • Rosie O'Donnell Marries Kelli Carpenter

    Rosie O'Donnell celebrates Newsom's order by marrying partner Kelli Carpenter. February 26, 2004.

  • War Protests

    Hundreds protest the War in Iraq. October 25, 2003.

  • City Hall Reopens

    After years of retrofitting after the earthquake of 1989, City Hall finally reopens. January 5, 1999.

  • Dianne Feinstein Wedding

    Then-mayor Dianne Feinstein marries Richard Blum at City Hall. January 20, 1980.

  • Dali Lama at City Hall

    The Dali Lama visits San Francisco City Hall. September 26, 1979.

  • Dan White Riots

    Demonstrators smash glass at City Hall in protest of the voluntary manslaughter conviction in the fatal shootings of Harvey Milk and George Moscone. May 22, 1979.

  • Harvey Milk Killed

    Officers remove the body of Harvey Milk from City Hall after the fatal shootings that killed both Milk and George Moscone. November 27, 1978.

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