On August 9, 2019, about 1820 Pacific daylight time, a Cessna 206, N5152U, was substantially damaged when it was involved in an accident near Prospect, Oregon. The pilot and passenger were not injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 135 fire reconnaissance mission flight. According to the pilot, during takeoff and climb, he noted that all gauges indicated normal; he checked the gauges every 10-15 minutes during the flight. About 2.8 hours into the flight, over Rustler Peak, they heard a loud bang followed by violent shaking of the airplane. The pilot believed the airplane was at an altitude of about 1,500-2,000 ft. above ground level (agl). The pilot retarded the propeller control to extend his glide distance while he looked for a place to land. He then radioed air attack, who relayed a mayday call. The pilot stated that he found an open area to land, activated the emergency locator transmitter (ELT), switched the radio to 121.5 and made a mayday call. The open area where he landed was a swamp area and as soon as they touched down, they were thrown forward in their seatbelts. They exited the airplane and waited to be rescued. A complete engine teardown inspection was performed after the accident. Numerous metal pieces and shavings were noted in the oil pan including: a broken counterweight, crankshaft counterweight attachment, counterweight retainer pin, damaged retainer pin plates, damaged retainer plate spring clips, and engine casing pieces. One of the damaged retainer pin plates displayed a deep groove, the approximate diameter of a counterweight pin. Additionally, extensive damage was noted to the no. 1 cylinder, piston, and connecting rod. The no. 1 cylinder valve guide rods broke through the crankcase and displayed damage. The rear counterweight was separated from the crankshaft and the counterweight lobe was broken free from the crankshaft. Examination of the engine logbooks revealed the engine was last overhauled in April 1999. During that overhaul, the crankshaft was removed and replaced with the one involved in the accident. An entry dated March 5, 2010, indicated the engine was disassembled on that date and the components were inspected. According to the entry, the crankshaft assembly was magnafluxed and dimensionally inspected before being returned as serviceable. The work order associated with the March 5, 2010; maintenance entry indicated that the crankshaft was overhauled by ECi in June 2009. The crankshaft assembly accumulated 1,380.7 hours since that overhaul (as of the last maintenance entry 3 days before the accident).