Well, it's not actually so secret. The existence of the Hall of Records at Mount Rushmore National Memorial is well-known, at least among residents of the Black Hills. It's not like the National Park Service has been particularly sneaky about it, either: there are interpretive panels and exhibits at the memorial's museum that detail its backstory: why the mountain's sculptor conceived of it, what it was supposed to look like, and how it was finally completed in 1998.
But for some reason, multiple travel blogs and news outlets suddenly started writing about it last week. We're not sure who started the trend, but writers picked it up and ran with it. The Telegraph even did a whole series on hidden caves and rooms in popular American attractions.
Most of our guests make a trip to Mount Rushmore during their Black Hills adventures, but sadly, the Hall of Records isn't open to the public. If you're interested in knowing a bit more about it, though, ask the innkeepers at the 1899 Inn: when we were growing up (long before any security scares), we had family connected with the National Park Service, and we both were lucky enough to get a quick tour on top of the mountain - including back to the Hall of Records.