ROOF FALL WAS DISCOVERED WHILE EXAMINING AIRCOURSES. IT FELL IN AN INTERSECTION LOCATED IN THE RETURN AIR COURSES #2 ENTRY, XCUT 26, 1 RIGHT 2ND WEST. THE SIZE OF FALL WAS 40' X 18' X 6' HIGH. IT WAS CAUSED BY A HORSEBACK ROCK IN ROOF. NO ONE WAS INJURED, NO DAMAGE TO EQUIPMENT. DID NOT BLOCK AIR.