Dunderberg, which is a Swedish word meaning "thunder(ing) mountain," was an ocean-going ironclad screw frigate of 16 guns. She was designed by Lenthall as a reproduction of CSS Virginia, with two 21-foot screws, sloping armored casemate sides, and a 50-foot ram. She had a double bottom and collision bulkheads, and was the longest wooden ship ever built. Her keel was laid down in October 1862 by W.H. Webb of New York City. Her construction was initially spurred by the threat of war with England. After that impetus abated, construction lagged, and she was not launched until 2 March 1865. The American Civil War ended before she could be completed, and was formally rejected by the U.S. Navy in September 1866.
Webb began seeking buyers for the warship, the design of which was already beginning to influence naval architecture worldwide. Otto von Bismarck expressed some interest, and the thought of Prussia armed with such a vessel prompted France to hurriedly buy her and commission her in 1867 as Rochambeau. The French scrapped her in 1874.