The Rapid City Journal and Bismark Tribune have an article about some "artistic license" taken with the geography of the Black Hills in the new "National Treasure" movie:
If the sequence followed reality, the characters would find themselves in front of a nondescript tunnel carved 70 feet deep into the rock where Rushmore sculptor Gutzon Borglum once envisioned putting a "Hall of Records." Borglum's plan was to house historical documents such as the Constitution of the United States and the Declaration of Independence there and make it accessible to visitors.
The sculptor's plan was never completed, but the actual tunnel does feature a capsule sealed with a granite capstone holding including a biography of the artist, stories of his struggles to carve the memorial and other Mount Rushmore historical data.
The Hollywood version takes a different route.
Once the movie's treasure hunters climb the metal staircase, they wind up on a beautiful lake that actually doesn't exist at Mount Rushmore. The real location of the shot is Sylvan Lake, about 30 miles away in Custer State Park.
Sylvan Lake is perhaps the most beautiful lake in the Black Hills. The trailers I've seen which show Sylvan Lake look very breathtaking.
Even with the "liberties" taken with geography, I look forward to seeing all this fit together in the movie.