Mt. Rushmore may be one of the most recognizable landmarks on the planet, but it has been harboring a little-known secret for decades.

According to The Sun, this iconic stone sculpture, featuring the faces of US presidents George Washington, Thomas Jefferson, Theodore Roosevelt and Abraham Lincoln, contains a secret chamber which very few people know about.

The secret room can be found behind Abraham Lincoln's head, and was designed to serve as a Hall of Records to tell the complete story of the United States’ history.

During its construction, from 1927–1941, Gutzon Borglum, the sculptor who designed the moment had initially planned for the sculpture of each president to go down to their waists, but due to time and budget constraints they were only able to work on the heads.

After the main sculpting project was halted, the government gave Borglum the permission to start work on a Hall of Records – a hidden chamber packed full of America’s most important paperwork.

But Gutzon Borglum died before his record room could be completed, leaving the hall unfinished for decades.

However, in the late 1990s, the project was given a new lease of life, and the chamber room was finally completed, and was stocked with panels featuring the American history, Declaration of Independence, the Constitution, and the Bill of Rights.

A biography of Gutzon Borglum was also placed in the room, before it was all locked in a titanium vault.

There are hopes that, thousands of years from now, a future society will stumble across the vault and learn more about American history.

Tourists looking to visit the secret room will be disappointed however, as the chamber is far too difficult to reach on foot.