An explosive, 500-square-mile wildfire burned nearly to the doorstep of this evacuated mountain town on Monday as ash rained down on crews desperately fighting to hold back the flames. The largest fire in Arizona history burned to within half a mile of this town of 7,700 by Monday night. It had already destroyed at least 375 houses, most of them in Heber-Over-guard, a community 35 miles west of Show Low that burned over the weekend.