Who built Mt. Rushmore
Gutzon Borglum.
god
Mount Rushmore
Because they ran out of mountain to continue carving.
http://www.crazyhorse.org/
Mainly by carving it.
The carving of the faces on Mount Rushmore started in 1927, and ended in 1941.
There is no "secret room" on Mount Rushmore. There is what Gutzon Borglum had hoped would be a "Hall of Records". It was to be located in a canyon which is directly behind the faces. Carving was started on the entrance to the Hall of Records, but discontinued when Congress directed that construction was only to be done on the faces.
carving of mount Rushmore was complete
Carving began in 1927 and ended in 1941, so it took 14 years to carve Mount Rushmore.
Carving the figures into Mount Rushmore took 14 years, October 4, 1927 to October 31, 1941.
Bosun Chair
Mount Rushmore itself is not a museum. There are two museums on the grounds of the memorial. There is a Sculptor's Studio where you can see the tools and techniques used in carving the mountain. The Lincoln Borglum Visitor Center has photos and videos about the history of Mount Rushmore.
Nothing of any great historical importance happened at Mt. Rushmore, unless you consider the carving itself to historical.