On July 18, 2016, about 0930 central standard time, a Nelson Woody homebuilt, tailwheel-equipped, experimental airplane, N525AG, registered to the pilot, sustained substantial damage after veering off the runway during landing rollout at the Hope Municipal Airport (M18), Hope, Arkansas. The private pilot sustained serious injuries and one passenger sustained minor injuries. The personal flight was being conducted under the provisions of Federal Code of Regulations Part 91. Visual meteorological conditions prevailed throughout the vicinity and a flight plan was not filed. The flight originated from M18 about 0900. According to the pilot, he was conducting touch and go landings at M18 on runway 16. Upon a normal touchdown, the aircraft veered to the left off the runway surface. There were ground impressions of the left wingtip and propeller marks on the ground. Evidence showed that the airplane ground looped as it departed the runway. Both occupants exited the aircraft and the pilot was transported to a local hospital for his injuries. Examination of the airplane by an FAA inspector after the accident revealed that the left wing was bent downward mid-span about 90-degrees, and the fuselage was buckled. The left main landing gear retaining mechanism was found broken. It was not clear if the damage was pre or post impact. The pilot, who was the builder of the airplane and had about 500 hours of flight time in the airplane, reported that he believed that the retaining mechanism could have broken upon landing resulting in the ground loop. The nearest weather reporting station, located in Texarkana, Arkansas, about 30 miles from the accident site, reported clear skies and variable winds at 4 knots. Local personnel located at the airport reported that it was a nice day with light winds on the day of the accident.