CAR WAS PARKED ON LEVEL GROUND OUTSIDE HOME. ATV ACCIDENTALLY RAN INTO REAR BUMPER OF CAR (HAD MOVED APPROXIMATELY 10 FEET). CAR ROLLED AWAY AND ENDED UP ROLLING DOWN A SMALL HILL, JUMPING A DITCH AND RESTING ON ANOTHER HILL. CAR MOVED APPROXIMATELY 300 FEET PRIOR TO COMING TO A STOP. WHEELS WERE NOTED TO BE TURNING AND ROLLING NORMALLY AND CAR HAD MINIMAL RESISTANCE TO MOVING. CAR WAS IN PARK WITH SECURE BREAK WHEN WE ARRIVED AT CAR.