Popocatepetl volcano erupts with ash clouds in Mexico.

Mexico's Popocatepetl volcano has been spewing toxic fumes, ash, and lumps of incandescent rock persistently for almost 30 years, since it awakened from a long slumber in 1994. The volcano is 45 miles southeast of Mexico City, but lies much closer to the eastern fringes of the larger metropolitan area of 22 million people. A severe eruption could cut off air traffic or smother the city in clouds of choking ash. The volcano is closely monitored by a network of sensors and cameras, and a roomful of powerful equipment watching its every move. A volcano warning "stoplight" with three colors: green for safe, yellow for alert, and red for danger is used to warn people of an impending ash cloud.