Rock carvings portraying four US presidents at Mount Rushmore are being washed for the first time since they were completed more than 60 years ago.
Park rangers want to remove lichen that has been corroding the 18m-high sculpted granite faces in the state of South Dakota.
The carvings were created between 1927 and 1940 by Danish-American sculptor Gutzon Borglum and 400 hired labourers.
They portray Washington, Jefferson, Lincoln and Theodore Roosevelt.
Although the carvings are checked every year for cracks, they have never been cleaned of their six decades of layers of grime and dirt.
It is feared that some presidential facial features could eventually drop off unless the growing number of lichen plants are removed.
The Mount Rushmore park rangers have called in the expertise of a German company that has cleaned the statue of Christ above Rio de Janeiro and the Brandenburg Gate in Berlin.
For the next five weeks or so under the supervision of the Alfred Karcher Company, park rangers will be seen hanging precariously from rope harnesses, employing hot water blasters and silicone sealant dispensers.