Red HotOne of the world's most active volcanoes, Mexico's Popocatepetl volcano, has come to life, spewing ash, gas, and steam (as seen from the village of Santiago Xalitzintla on July 4).Just 50 miles (80 kilometers) east of Mexico City, the volcano has spit out a cloud of ash and vapor 2 miles (3.2 kilometers) high.The volcano has been exploding for several days, causing Mexico's National Center for Disaster Prevention to elevate their national alert rating to Yellow Phase 3. A yellow alert signals that the volcano has demonstrated medium-to-high levels of activity. The next and final step would be a red alert, which would require the evacuation of nearby residents. (Watch video: Volcanoes 101.)Al Jazeera reported that international airlines have cancelled flights in and out of Mexico City, stranding hundreds of people over the weekend.This is not the first time the 17,900-foot (5,455-meter) Popocatepetl has been active this year. In June, the warning level for the volcano was also raised to yellow after an explosion generated an ash plume that rose over a mile (1.6 kilometers) above the crater.—Jaclyn Skurie

Photograph by Francisco Guasco, European Pressphoto Agency