Hmmm…claims to hold historic relics, but could it be used for something else?
What's Inside Mount Rushmore's Not-So-Secret Chamber?
There is a 70-foot (21.4-meter) passageway carved inside the mountain, and while it's not quite top secret, the space is not open to the public
https://history.howstuffworks.com/american-history/whats-inside-mount-rushmores-not-so-secret-chamber.htm