Popocatepetl

Smoke rises from the Popocatepetl as it spews incandescent volcanic material on the outskirts of Puebla July 4, 2013. The 5,450-metre (17,900 ft) Popocatepetl volcano has been kept on yellow phase two alert, the second-highest warning on the center's seven-step scale, according to local media.

melda Medina/Reuters

Smoke rises from the Popocatépetl volcano as it spews incandescent volcanic material on the outskirts of Puebla, Mexico on Thursday.