On August 17, 2003, at 1200 eastern daylight time, a Lake 250 amphibian airplane, N8550U, was substantially damaged during landing on Center Pond, Phippsburg, Maine. The airline transport pilot and passenger were not injured. Visual meteorological conditions prevailed and no flight plan was filed for the personal flight conducted under 14 CFR Part 91. According to the pilot, he overflow the pond prior to landing, and did not observe any boat traffic, or debris on the water. He performed one touch-and-go landing, and returned for a second landing. The touchdown was "normal," and during the landing rollout the airplane started to come off the step and began to turn to the left, "like it had hooked something." The pilot taxied to the shore and docked the airplane, at which time he realized he "must have hit something on the bottom of the pond," since there was mud in the lower nose gear doors. According to a Federal Aviation Administration inspector who examined the airplane, the left wing, left wing spar, and left sponson sustained substantial damage. No mechanical deficiencies were noted.