Threatening 22 million people, Mexico's Popocatepetl is a very closely watched volcano

Mexico’s Popocatepetl volcano rumbled to life again this week, spewing out towering clouds of ash that forced 11 villages to cancel school sessions. Because it looms so close to the Mexico City region, the volcano's every sigh is closely monitored by dozens of scientists, a network of sensors and cameras and a roomful of powerful equipment ready to sound warnings. The 17,797-foot mountain is known affectionately as “El Popo” and has been belching toxic fumes, ash and lumps of incandescent rock persistently for almost 30 years. The volcano is just 45 miles southeast of Mexico City, but it stands much closer to the eastern fringes of the metropolitan area, which is home to 22 million people. A severe eruption could smother the city in clouds of choking ash.