On July 5, 2006, about 1100 Pacific daylight time, a Cessna 210K, N12PF, veered off the runway and impacted a ditch during landing at Gnoss Field Airport, Novato, California. The owner/pilot was operating the airplane under the provisions of 14 CFR Part 91. The commercial pilot and one passenger were not injured; the airplane sustained substantial damage. The cross-country personal flight departed San Carlos, California, about 1020. Visual meteorological conditions prevailed, and no flight plan had been filed. The National Transportation Safety Board investigator-in-charge interviewed the pilot on the telephone. The pilot reported that while on the downwind to land on runway 13 she and her passenger remembered hearing the gear extending, and she confirmed the landing gear indicator light was illuminated. After touchdown the airplane veered to the left, departed the runway, and came to rest in a ditch. A witness to the accident said that he saw the airplane on final approach and the left landing gear was not in the locked position and was trailing below the airplane. During recovery process it was noted that both the nose gear and the right main gear were in the extended and locked position. The left main gear was in the wheel well and was manually pulled out of the wheel well and locked into the down position. The airport personnel reported that the tire marks on the runway indicated that the left main gear had not supported any substantial weight of the airplane during the landing.