<p><strong>One of the world's most active volcanoes, <a href="http://travel.nationalgeographic.com/travel/countries/mexico-guide/">Mexico</a>'s Popocatepetl <a href="http://environment.nationalgeographic.com/environment/natural-disasters/volcano-profile/">volcano</a>, has come to life, spewing ash, gas, and steam (as seen from the village of Santiago Xalitzintla on July 4).</strong></p><p>Just 50 miles (80 kilometers) east of <a href="http://travel.nationalgeographic.com/travel/city-guides/mexico-city-mexico/">Mexico City</a>, the volcano has spit out a cloud of ash and vapor 2 miles (3.2 kilometers) high.</p><p>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. (<a href="http://video.nationalgeographic.com/video/environment/environment-natural-disasters/volcanoes/volcanoes-101/">Watch video: Volcanoes 101</a>.)</p><p>Al Jazeera reported that international airlines have cancelled flights in and out of Mexico City, stranding hundreds of people over the weekend.</p><p>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.</p><p><em>—Jaclyn Skurie</em></p>