Biden, Harris lay wreath at Tomb of the Unknowns in Arlington National Cemetery

President Joe Biden and Vice President Kamala Harris attended a wreath-laying ceremony after the inauguration Wednesday at Arlington National Cemetery's Tomb of the Unknown Soldier, a tradition among incoming administrations.

Arlington National Cemetery is the final resting place of more than 400,000 men and women, including U.S. presidents, flag raisers pictured on Iwo Jima, and Civil War generals.

ARLINGTON, VA - JANUARY 20: U.S. President Joe Biden and Vice President Kamala Harris attend a wreath-laying ceremony at Arlington National Cemetery's Tomb of the Unknown Soldier after the 59th Presidential Inauguration ceremony at the U.S. Capitol J

The monument’s history dates back to 1921, when Congress approved the burial of an unidentified American soldier from World War I in the plaza of Arlington National Cemetery's new Memorial Amphitheater.

The WWI "Unknown" arrived in Washington, D.C. in November of that year and lay in state at the Capitol Rotunda — with thousands of visitors lining up to pay their respects, according to Arlington National Cemetery's website.

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This led Congress to establish a military guard to protect the tomb. And since July 2, 1937, those interred at the Tomb of the Unknown Soldier, also known as Tomb of the Unknowns, have never been alone.