Mount Rushmore
This iconic U.S. landmark has a hidden feature. Behind the head of Abraham Lincoln, there is a secret door. The sculptor who designed Mount Rushmore, Gutzon Borglum, wanted the monument to include a written description of the nine most important events in U.S. history. However, his plan was too intricate and he was only approved to work on a Hall of Records (the hidden room). He died before it was completed, but copies of the Declaration of Independence, Constitution, and the Bill of Rights are on display in the room today. Very few people have actually been able to visit this room, though, because it's difficult to reach by foot. If you decide to go, here's your guide to a Mount Rushmore trip.
8 Famous Monuments That Are Hiding Little-Known Secrets, Source:https://www.rd.com/list/monument-secrets/