AFTER OVERSEEING THE ACTIVITIES OF CONTRACTORS WORKING ON THE BELT 614 LEVEL, EMPLOYEE LEFT AND WENT UP THREE FLIGHTS OF STAIRS TO THE 612 BELT LEVEL WHERE THE ELEVATOR TO THE SURFACE WAS LOCA TED. WITHIN A MINUTE OR TWO OF LEAVING, HE FELL THROUGH A 4X4 OPENING THAT HAD BEEN USED TO LOWER PARTS. THE HOLE HAD FLAGGING AT ALL ENTRIES AT LEAST 4' FROM THE EDGE. HE FELL 19' TO THE BEL