PARKED VEHICLE ON A SLOPE IN THE MOUNTAINS WITH THE PARKING BRAKE ENGAGED. THE VEHICLE IS AN AUTOMATIC TRANSMISSION AND WAS IN PARK. IMMEDIATELY FOLLOWING ALL PASSENGERS EXITING THE VEHICLE, IT SLOWLY STARTED ROLLING AWAY, BACKWARDS. IT TRAVELED DOWN A 4WD ROAD A SMALL DISTANCE BEFORE TURNING OFF AND TUMBLING DOWN THE MOUNTAIN. LUCKILY EVERYONE HAD EXITED THE VEHICLE AND THERE WERE NO INJURIES. THOUGH WE WERE PARKED ON A SLOPE, PROBABLY AROUND 15 DEGREES, THERE WAS COMPLETE FAILURE OF BOTH THE PARK GEAR AND THE PARKING BRAKE ALLOWING A ROLLAWAY. THE PARKING SURFACES WAS A COMBINATION SOLID ROCK AND DIRT.