WE EXPERIENCED A FALL IN THE INTERSECTION OF #3 ENTRY OF THE 1ST NORTHWEST SUB-MAINS 3 BREAKS INBY SPAD #681. IT MEASURED 35'LX30'WX4' TO 7' THICK. NO EQUIPMENT OR PERSONNEL INVOLVED. FALL OCC URRED IN AREA-SUPPORTED WITH 5 FOOT, FULL-GROUTED TENSION RE-BAR ROOF BOLTS. FALL WILL NOT BE CLEANED UP.