Mount Rushmore, with the secret room in view just behind Lincoln's head. Pinterest Since taking office, President Donald Trump has reignited a debate about what it means to be American.

But many Americans might not know the secret behind one of the country's most iconic political monuments.

Enter: The Hall of Records at Mount Rushmore.

Where the frontal lobe of Abraham Lincoln's brain would be, there is a secret room that contains the text of America's most important documents. It sounds like something out of "National Treasure" (or, more fittingly, "Richie Rich") but the Hall of Records is a legitimate historical repository.

Conceived in the 1930s by the monument's designer, Gutzon Borglum, the Hall was designed to be a vault for a selection of documents chronicling America's history.

Inside the entrance to the Hall of Records. National Park Service Borglum envisioned an 800-foot stairway leading to a grand hall, measuring 80 feet by 100 feet, behind the presidents' faces. Above the entrance to the hall would hang a bronze eagle, with a wingspan of 38 feet. The hall would contain busts of famous Americans and a list of US contributions to science, art, and industry, according to the National Park Service.

Unfortunately, Borglum died in 1941, so he never got to see that vision come to life. More than 50 years later, in 1998, monument officials revived the sculptor's dream of installing a record of the country's history inside the Hall.

Today, sealed behind a 1,200-pound granite slab and tucked inside a wooden box are the US Constitution, the Declaration of Independence, the Bill of Rights, a biography of Borglum, and short descriptions of each of the presidents featured on the monument. The text of each document is carved into a series of porcelain enamel panels.

Sadly, the Hall is closed to the public (the half-ton slab probably already gave that away). The closest anyone can get is the ruin-like doorway, which recedes several feet into the mountain. It's nestled behind a rocky outcropping to the right of Lincoln's head.

Inside the entrance, looking out. National Park Service The present-day Hall doesn't contain any of the intricate designs Borglum originally envisioned for its walls: carved descriptions of America's greatest moments, surrounded by massive illustrations of the Louisiana Purchase.

At least, as far as we know. Maybe Nicolas Cage knows something we don't.