THE INJURED WAS OPERATING THE RADIO CONTROLLED CONT MINER WHEN A PIECE OF ROOF ROCK FELL OUT FROM BETWEEN THE RIB BOLT AND THE RIB STRIKING HIM ON THE BACK OF BOTH LEGS JUST BELOW THE KNEES. THE ROCK MEASURED 6 FT LONG BY 18 INCHES WIDE BY 4 INCHES THICK, RESULTED IN A BROKEN TIBIA OF HIS LEFT LEG.