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.

Mammoth Cave in Kentucky is the longest cave system on Earth - over 400 miles of tunnels. In 1842, Dr. John Croghan moved 16 sick patients underground, convinced that cave air would cure their illness. At least five died inside. Nearly all the others died within weeks of leaving. The doctor died of the same disease in 1849.

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.

Betty Willis designed the "Welcome to Fabulous Las Vegas" sign in 1959 and never copyrighted it. She called it her gift to the city, leaving the design free for anyone to use. That is why it appears legally on T-shirts, postcards, and souvenirs around the world with no royalties owed. She once said: "I should make a buck out of it. Everybody else is."

The Lincoln Memorial hid a secret room for over 100 years. It just opened to the public for the first time. The new $69 million museum has floor-to-ceiling glass walls. Visitors can walk past the columns holding up the memorial above. Lincoln's own signed Emancipation Proclamation and 13th Amendment are inside.