During the construction of this famous monument, architect Gutzon Borglum wanted to create a Hall of Chronicles inside the cliff face. For this purpose, he excavated a cave behind the head of Abraham Lincoln. In 1998, more than 50 years after his death, copies of important documents and the memoirs of various presidents were placed inside the unfinished hall, which now serves as a time capsule. Nobody knows how the cave was used, remaining empty for 50 years. Some people think there were some secret laboratories. Who knows?