SANTIAGO XALITZINTLA, Mexico: At the edge of this town near the Popocatepetl volcano, away from the din of traffic, there was an occasional low rumble Monday, like an idling engine.

A cloud of superfine ash descended, slightly reducing visibility and coming to rest on vehicles' windshields. For more than a week, the 17,797-foot (5,425-meter) mountain just 45 miles (about 70 kilometers) southeast of Mexico City and known affectionately as 'El Popo,' has been increasingly explosive, spewing great plumes of gas, ash and incandescent rock into the air.