The pilot said that he made low passes over the flat, tundra site to assess its suitability for landing. He said that, on the second pass, he touched the airplane's wheels down to verify that the surface was firm and then returned for landing. He said that both visually and during the test-touch, the surface appeared to be "100 percent hard-surface tundra," and the vegetation appeared dry; however, during the end of the landing roll, when the airplane was traveling about 10 mph, the wheels sank into unexpectedly "mushy" ground, and the airplane slowly nosed over. The pilot reported no mechanical malfunction that would have precluded normal operation. Damage to the airplane included rudder, strut, and wing rib damage. The airplane was equipped with tundra tires. Photographs of the airplane at the accident site showed it inverted in a broad area of flat, grassy tundra, consistent with the pilot's description.