The sport pilot had just departed in his single-engine experimental airplane on a local flight when he experienced a total engine failure at an altitude of 500-700 feet. The pilot was unable to land on the remaining length of runway so he made a forced landing to a plowed field directly off the end of the runway. The airplane sustained damage to all three landing gear, the fuselage, and both wings. An examination of the engine revealed that a source of the power loss could not be determined. No other anomalies were found with the airplane.