In July 1938, some 11 years after first starting work on Mount Rushmore, artist and sculptor Gutzon Borglum shifted attention to a critical detail of the monument that neither he nor the American public would ever see completed.

In a canyon concealed behind the massive heads of George Washington, Abraham Lincoln, Thomas Jefferson and Theodore Roosevelt, Borglum instructed workers to start cutting a vault into the solid granite walls. Measuring 18-feet tall, this opening would serve as the entrance to Borglum's "Hall of Records," a repository intended to not only tell the story of the monument, but serve as an everlasting time capsule for some of American history.

“Into this room the records of what our people aspired to and what they have accomplished should be collected and preserved," Borglum wrote, "and on the walls of this room should be cut the literal records of the conception of our republic, its successful creation, the record of its westward movement to the Pacific, its presidents, how the memorial was built and, frankly, why."

The original plans for Mount Rushmore's Hall of Records as conceived by Gutzon Borglum. (Photo: Mount Rushmore National Memorial)

As you might expect, Borglum's plans for the Hall of Records were extremely ambitious. Visitors would ascend an 800-foot granite stairway from the base of the monument to the hidden mouth of the canyon behind Lincoln's head. After passing through the entrance, they would reach an elevated section flanked by cast glass doors and featuring a bronze eagle with a wingspan measuring 38 feet. According to the National Parks Service, the words inscribed over the eagle were to read "America’s Onward March" and "The Hall of Records."

The chamber itself, featuring bronze-and-glass cases containing historical documents, as well as busts of other notable Americans, was to measure 80-by-100 feet.

Workers in 1938 carving out the initial 18-foot-tall entrance to Mount Rushmore's Hall of Records. (Photo: National Park Service)

According to historian Amy Bracewell, Borglum's team spent almost a year working on the vault – carving out an initial tunnel nearly 70 feet deep. Unfortunately, the sculptor apparently neglected to inform Congress of his plans for the massive chamber. Concerned with increasing costs, they immediately asked that he cease efforts on the Hall of Records and refocus on finishing the presidential faces.

After Borgman passed away in March 1941, work on Mount Rushmore wound down and the unfinished Hall of Records became something of a concealed secret. While his grand scheme for the space was never realized, his family did succeed in fulfilling at least part of the dream. In 1998, monument officials joined four generations of the sculptor's family in entombing a record of America within the vault's granite walls. Under a 1,200-pound capstone, they placed 16 porcelain panels into a teakwood box printed with historical documents, images, and information on the monument's creation.

The capstone marking the site of the buried porcelain records at the entrance to the unfinished Hall of Records. (Photo: Wikimedia Commons)

Inscribed on the capstone is a quote from Borgman's 1930 dedication of Washington's head on Rushmore.

"... let us place there, carved high, as close to heaven as we can, the words of our leaders, their faces, to show posterity what manner of men they were. Then breathe a prayer that these records will endure until the wind and rain alone shall wear them away."

As you might expect, the Hall of Records is not accessible to the public today. The entrance is located near the monument's steep cliffs (and that 800-foot granite staircase was never built), so safety is likely the biggest reason glimpses of this hidden chamber's existence continue to elude visitors.