The Popocatepetl volcano that towers over Mexico City began rumbling again today, shooting a blast of ash about 2 miles above its crater at dawn. The 17,886-foot mountain, nicknamed "Popo," shook for several minutes before the ash burst out. People were urged to stay at least 7 miles from the crater, which is about 40 miles southeast of Mexico's capital. The volcano is thought to be stable, but this is the biggest such cloud in several years.