Monuments 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.

The Gateway Arch was built as two separate legs measured to meet precisely at the top. On October 28, 1965, the morning sun expanded the south leg. The last piece would not fit. Workers ran fire hoses to the top and soaked the steel until it shrank. Hydraulic jacks applied 450 tons of force, and the final section slid in.

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.