Hidden History Facts

The Lincoln Memorial has a secret: a 43,800 sq ft cave beneath it - nearly twice the floor space of the memorial above. Rainwater seeping through the marble has grown real stalactites, and 1914 construction workers left cartoon graffiti on the columns, now preserved behind plexiglass. It opens as a public museum for the first time on June 25.

Michelangelo was sentenced to death in 1530. He hid for two months in a secret coal vault, drawing on the walls. Those drawings - attributed to him but still debated - lay plastered over for 445 years. In 1975, a director found them under a trapdoor. They are still on the walls.

Mount Rushmore has a secret: a 70-foot granite tunnel cut directly behind the faces. Sculptor Gutzon Borglum began drilling it in 1938 to house America's founding documents for future civilizations. Congress shut the project down in 1939 and Borglum died in 1941. In 1998, his family lowered 16 porcelain panels into a titanium vault and sealed it under a 1,200-pound granite capstone. Nobody can go inside.