South Dakota's most famous feature is without a doubt Mount Rushmore and can be easily spotted on any license plate or state tourism literature. Sculptor Gutzon Borglum, along with 400 workers carved the granite mountain over a period of 14 years from 1927 to 1941 into the memorial we see today.
Commissioned in 1925, the memorial was to embody the vision of the leaders of the United States through it's first 150 years as a nation. George Washington, Thomas Jefferson, Abraham Lincoln, and Theodore Roosevelt stare off into the Black Hills of South Dakota. Gutzon Borglum selected Mount Rushmore because of it's ideal location, and the quality of the stone in the mountain. The original vision was to include the chests of the presidents, but a number of issues during the carving scaled back the plans.
The worst of these set backs involved blasting one of the faces off to start completely over after two years of work! The order of the presidents and how they faced also had to be modified during the 14 years of work to accommodate problems with the rock. In the end the project had an impressive track record. Not a single worker was killed working on the carving and there is only one visible mistake. A drill bit became stuck in the rock, and instead of risking causing more damage, it was decided to leave it in, leaving a small visible rust streak now evident on the memorial.
Mount Rushmore has gone through a series of repairs over the last fifty years. There are some visible cracks in the face of the memorial and recently these cracks were filled with silicone based material to keep water out. This technique of preservation has been used for over 30 years in the White Mountains of New Hampshire to save the Old Man On The Mountain from similar damage. The endless cycle of freeze and thaw that happens in the Black Hills would eventually destroy the memorial, but the recently completed sealing and repairs should keep it around for thousands of years.
The National Park Service also recently stored a time capsule at the memorial. In the event that our civilization is every destroyed, the capsule explains why Mount Rushmore is there, who the people are, and why they were carved.