On June 1, 1994, at 1400 Alaska daylight time, a wheel equipped Piper PA-12 airplane, N4380M, hit a 3-4 ft. deep trench during landing rollout, collapsing the landing gear, and damaging the wings and fuselage. The personal flight was conducted under 14 CFR Part 91 in visual meteorological conditions. The flight departed from Fairbanks and the destination was a sand bar on the Tanana river. No flight plan was filed. The Pilot in Command was uninjured. The aircraft was substantially damaged. According to the Pilot in Command, he was looking for a place to land and made two passes, one at 200 feet above the ground (AGL) and another at 150 AGL. He then made two more passes perpendicular to the intended landing site at 50 AGL. He stated he could see no obstruction. He lined up on final and intended to do a "3 foot drag" and possibly touchdown. He touched down and stated the landing was smooth and he decided to brake and remain on the ground. Approximately 2/3 of the way down the landing area he saw a ditch and was not able to execute a go around or stop before hitting the ditch. When the airplane struck the ditch, the main landing gear separated from the fuselage and the airplane skidded to a stop while dragging its right wing.