On October 5, 1999, about 1952 hours Pacific daylight time, a Cessna 210B, N9609X, sustained substantial damage when it collided with terrain while on a left base leg for landing on runway 15 at Tonopah, Nevada. The pilot operated the airplane under the provisions of 14 CFR Part 91. The commercial pilot and one passenger sustained serious injuries. The personal flight originated in Yelm, Washington, with stops in Chehalis, Washington, and Klamath Falls, Oregon. The pilot departed Klamath Falls at 1737. Visual meteorological conditions prevailed and no flight plan was filed. The airport operator reported the airplane requested advisories on the Unicom frequency. The runway lights illuminated and the operator awaited the airplane's arrival. When the airplane did not appear, and the runway lights shutoff automatically, the operator began looking for the airplane. He saw a rotating red light in the area where the base leg should have been flown, drove to the site, and discovered the airplane. He reported the two occupants were somewhat dazed, suffered cuts and bruises, and were transported to the hospital. The operator turned the fuel switch from right to off, and turned the electrical master switch off. He noted it was a very dark night. The pilot stated the ground level at the point of contact was higher than the runway threshold and the threshold lights were not in view. He said there was no lights east of the runway to reference once the airplane passed abeam the threshold lights on the downwind leg until the threshold lights were reacquired. In his written statement, the pilot noted, "Apparently excessive altitude was lost while on downwind." During the base turn, the left wheel contacted the ground and rolled for a short distance before collapsing as the airplane slid to a stop.