XALITZINTLA, Mexico (AP) — 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 (5,450-meter) 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 on Friday, the mountain moved into what appeared to be a new level of activity, spitting out dozens of ash and shot 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 (1 million cubic meter) chamber of magma is seething about six miles (8 to 10 kilometers) beneath Popocatepetl, a mountain named for a legendary indigenous warrior, Roberto Quaas, director of the disaster prevention center, told a press 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.

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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."

However, he said, "we know that this lava dome, sooner or later, will be destroyed by internal pressure."

Scientists have detected fracturing about 3.5 miles (5 to 6 kilometers) down, accompanied by small earthquakes measuring about 3.4 on the Richter scale, he said. They are monitoring the shaking and the gas and ash put out by the volcano so that

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

"These are figures that obviously alarm and concern us," Quaas said.

Gregorio Fuentes Casquera, the assistant mayor of Xalitzintla, a village of 2,600 people about seven miles (12 kilometers) from the summit, said the town had prepared 50 buses and was sending out its six-member police forces to alert people to be ready to evacuate.

"Everyone needs to take this seriously. This buzzing, this roaring isn't normal," he said, adding that he believed about half the populace would be willing to evacuate, while the rest would want to stay.

As the quiet of the corn fields and fruit orchards was pervaded by the volcano's spooky roaring, dozens of women lined up in Xalitzintla's main square to get free face masks and bottles of water. Health authorities were giving out 10 masks and 10 bottles of water to each family, and the surgical-style masks, intended to filter out the fine ash released by the volcano, were becoming common among the town(asterisk)s students, who are required to wear them in school. Few adults wore them.

"Right now we're not scared. When it's scary is at night, when it's putting out lava," said Nancy Agustin Inclan, 14, as she removed her mask and took a break outside the gate of the town's middle school.

President Felipe Calderon said live on national television that authorities are keeping open roads around the mountain, preparing emergency shelters and making sure residents know the latest information about a potential eruption.

Leon Analco Analco, 83, chopped corn stalks with a machete as he related some residents' plans to simply move to a shelter they'd constructed on a ridgeline about 200 feet higher than the rest of the village.

"This is a ravine. All the mud will run down here. It's dangerous," he said.

Webcam images on the site of the National Disaster Prevention Center showed the plume rising from the top of the peak at dawn, though clouds obscured the volcano for people further away. The Televisa television network broadcast images of red, glowing material rising from the crater and falling on its slopes.

Authorities this week raised the alert level due to increasing activity at the volcano, whose most violent eruption in 1,200 years occurred on Dec. 18, 2000.

The ash was blowing to the northeast, in the general direction of the city of Puebla.

Residents of Huejotzingo, 18 miles (30 kilometers) from the peak, went about the day as normal despite a magnificent plume of ash and water vapor. Luz Maria de Olate, 35, put her 5-year-old in a surgical mask because the teachers said the ash could damage children's lungs.

But like hundreds of other residents on the flank of the volcano, she doesn't fear an eruption. Neither did her son as they headed for school.

The recent round of eruptions, which began in 1994, ended a period of decades in which the volcano was seen as peaceful, sleeping giant, and a tourism attraction.

Epifanio Inclan, 77, a farmer, recalled guiding tourists to the very crater of the volcano in the 1950s.

"They paid us a peso or two, and a peso meant something back then," he said. Making the day-long climb in huaraches, the young guides would play soccer near the top of the peak while tourists descended into the crater.

"I'm not afraid. He's my uncle," said Oscar Olate, expressing the personal relationship many in the Mexico Valley feel toward Popocatepetl, whose name means "smoking mountain" in the indigenous Nahuatl language of the Aztecs.

People view it with fondness, said street vendor David Gorzo Navarro, 45.

"When the ash falls, it's like fertilizer," he said.

More than 30 million people live within view of the volcano, which sits at a point where the states of Mexico, Puebla, and Morelos come together.