The pilot and a passenger flew to a privately-owned airport. The pilot circled the airport and made a low pass to view the runway condition. The pilot noticed the turf runway had not been mowed recently and consisted of tall grass; however, he did not see any standing water. After the pilot landed and rolled down the runway, he encountered a soft surface with standing water, which was not visible from the air. The airplane’s main gear sank into the soft surface which allowed the airplane to nose over and came to rest on the top of the wings and vertical stabilizer. Both occupants were able to exit the airplane with no injuries. The airplane received substantial damage to the left wing spar and vertical stabilizer. The pilot reported no mechanical deficiencies with the airplane prior to the accident.