On July 11, 2021, about 1230 Alaska daylight time, a Cessna 170B, N2583C, sustained substantial damage when it was involved in an accident near Rampart, Alaska. The pilot was not injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight. The pilot initially reported that, while enroute over mountainous terrain, the engine began to “run rough.” He looked for an emergency landing area, observed a “flat” spot, and maneuvered for landing. During landing, the main landing gear touched down and the terrain “immediately sucked [the airplane] in.” The pilot classified the terrain as very soft and full of moss and bumps. The airplane nosed over and came to rest inverted, resulting in substantial damage to the left wing and empennage. Responding law enforcement reported that the airplane was found upside down on top of a mountain in an area of knee-high clumps of grass. There were parallel tire marks leading up to the airplane for approximately 120-150 yards and disturbed vegetation where the tires made hard contact just before the airplane flipped. The pilot reported he had damaged his airplane while attempting to land and stated that “he was testing the surface to see if it was suitable for landing.” Postaccident examination of the airplane and a test run of the engine on the airframe revealed no evidence of any preaccident mechanical malfunctions or failures that would have precluded normal operation.