XALITZINTLA, Mexico – The white-capped volcano that looms over Mexico City emitted a terrifying low-pitched roar Friday and spewed roiling towers of ash and steam as it vented the pressure built up by a massive chamber of magma beneath its slopes.

Authorities prepared evacuation routes, ambulances and shelters in the event of a bigger explosion.

Even a large eruption of the 17,886-foot cone of Popocatepetl is unlikely to do more than dump ash on one of the world’s largest metropolitan areas. But the grit could play havoc with Mexico City’s busy airport and force the evacuation of tens of thousands of people in the farming villages on its flanks.

Popo, as it’s commonly known, has put out small eruptions of ash almost daily since a round of eruptions began in 1994. A week ago, the eruptions started growing larger and authorities slightly elevated the alert level for people living nearby.

Before dawn Friday, the mountain moved into what appeared to be a new level of activity, spitting out ash and shooting fragments of glowing rock down its slopes, and frightening the residents of surrounding villages with deep roaring not heard in a decade.

Residents of the village of Xalitzintla said they were awakened by a window-rattling series of eruptions.

Mexico’s National Disaster Prevention Center said that a string of eruptions had ended in the early morning, then started up again at 5:05 a.m., with at least 12 in two hours.

“Up on the mountain, it feels incredible,” said Aaron Sanchez Ocelotl, 45, who was in his turf grass fields when the eruptions happened. “It sounds like the roaring of the sea.”

A 35 million cubic foot chamber of magma is seething about six miles beneath Popocatepetl, a mountain named for a legendary indigenous warrior, Roberto Quaas, director of the disaster prevention center, told a news conference laying out emergency preparations.

Scientists have no way of predicting whether the molten rock in the chamber will be slowly released, or erupt in a powerful explosion like one on Dec. 18, 2000 that sent up a plume of red-hot rock and forced the evacuation of thousands of people who live at the volcano’s base.

He compared the volcano to a bottle of champagne, saying “you could take the cork out quickly and all the gaseous material and liquid rushes out suddenly, or it could also happen slowly.”

Scientists have detected fracturing about 3.5 miles down, accompanied by small earthquakes measuring about 3.4 on the Richter scale, he said.

An iconic backdrop to Mexico City’s skyline on clear days, Popocatepetl sits roughly halfway between Mexico City and the city of Puebla — meaning some 25 million people live within a 60-mile radius of the volcano, Quaas said.