One of the world’s most dangerous volcanoes may be showing signs of a massive eruption.

The alert level of the Popocatepetl volcano, which towers roughly 18,000 feet over sea level and looms 34 miles southeast of Mexico City, threatening 25 million people within range of an eruption, was raised to yellow phase three on Sunday, one step below a red alert. A yellow phase three alert level is triggered when the volcano shows “significant explosions of increasing intensity that shoot fragments (of rock) over considerable distances.” On Saturday the airports in Mexico City and Puebla temporarily halted flights because of ash spewed from the volcano.

“There is no risk to the population at this time,” Mexican national Civil Defense Coordinator Laura Velázquez said, claiming that the recent activity had been superseded in the past 20 years.

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Popocatepetl, which is Aztec for “smoking mountain,” had lain dormant for decades before it awakened in 1994 spewing gas and ash as far as 16 miles away by the winds. Prior to that eruption, the last explosion occurred in 1947. “In March 1996, a new round of activity began with increased ash emissions and the growth of a lava dome in the summit crater,” the U.S. Geological Survey noted.

Read more: Daily Wire