This is something the average tourist never gets to see. This is directly behind the faces and is called the Hall of Records. Gutzum Borglum (the sculptor of Mt. Rushmore) originally intended for this to hold other sculptures and records that detailed all about what Mt. Rushmore was, who carved it, why they did it, etc. for a time when this no longer exists. Well, Congress was paying for all this, and said they couldn't fund the Hall of Records as it was 1941 and WW 2 was going on, so it was never finished. There is a granite marker that was put in the Hall of Records a few years back that holds detailed records about Mt. Rushmore, so it fulfills hopefully some of what Borglum wanted put in there.