On September 15, 2005, about 1515 central daylight time, a Cessna 180A, N392PT, piloted by a private pilot sustained substantial damage during landing on a lake near Lodi, Wisconsin. The airplane was equipped with amphibious floats. The 14 CFR Part 91 personal flight was operating in visual meteorological conditions and was not on a flight plan. The pilot received minor injuries. The flight originated from Boscobel, Wisconsin about 1420. The pilot reported that he had made two landings on the lake prior to the accident. He stated that he made a third landing and after touchdown while the airplane was still "on the step", the airplane made a "hard" right turn then rolled left. The airplane came to rest inverted in the lake. The pilot stated that there were no wind gusts and that the manually pumped landing gear was not cycled between the three landings. The pilot stated that he believes the accident was the result of striking a submerged object during the landing. Examination of the aircraft by a Federal Aviation Administration inspector revealed no pre-impact anomalies. The floats were examined for evidence of damage due to impact with a submerged object. No damage consistent with striking a submerged object was found.