The pilot reported that he was landing on a private grass airstrip and that the grass was wet from the previous night’s rain. He stated that the airstrip was not flat and had sections that rolled up and down. He added that the airplane touched down harder than usual, which caused the landing gear to spread outward on the wet grass. The engine’s propeller was then low enough to contract the rising terrain and the airplane nosed over. The airplane came to rest in the inverted position, resulting in substantial damage to the vertical stabilizer, top wings, and the engine firewall. A witness reported that the airplane bounced on the runway about three times, before the airplane nosed over.