Washington (CNN) It's one of the nation's most important founding documents but the Declaration of Independence -- the document that paved the way for the creation of the United States -- hasn't always been treated with the care you might expect.

Since it was signed in 1776, the Declaration was haphazardly transported around the country in carriages, a sailing ship and even an unprotected car.

"It was terrible," said David Ward, senior historian at the National Portrait Gallery. "It was put in a burlap bag. They rolled it up all the time, which you're not supposed to do. It was exposed to light, it was exposed to smoke."

And at some point along the way, someone slapped a big, dirty handprint on the bottom left-hand corner—and never fessed up to it.

The dark smudge on the nation's famed document has baffled historians for years.

Read More