A FALL OF MINE ROOF OCCURED IN AN OUTBY RETURN AIR COURSE MINED IN 1999. IT FELL IN A CROSSCUT.IT WAS 35' LONG X 18' WIDE X 5' HIGH. THE CROSS-CUT HAD A SLIP RUNNING DIAGONALLY THAT WAS NOT DETECTED UNTIL IT FELL. IT DID NOT BLOCK PEOPLE,AIR OR EQUIPMENT.