Taal Volcano in the Philippines has been erupting for three days, blasting steam, ash and debris into the sky. And it shows no signs of stopping.

Experts tell TIME that they are watching closely for signs that a larger eruption could be brewing. Taal has a history of deadly explosions and is one of the more active explosive volcanoes in the world.

On Tuesday morning, approximately 50 earthquakes were detected in the region near the Taal Volcano, which is about 40 miles south of Manila. That could mean magma is still rising, according to the Philippine Institute of Volcanology and Seismology.

The picturesque volcano came to life on Sunday with a violent eruption that shot debris up to 9 miles into the air, prompting tens of thousands to evacuate and the closure of Manila’s Ninoy Aquino International Airport. Lava continued to gush out of the volcano on Monday and clouds of ash blew more than 60 miles north, reaching the Manila region, which is home to 12.9 million people. Schools and many offices closed.

The eruption of Taal volcano, south of Manila, Philippines, in an image made available by Himawari-8 satellite via the National Oceanic and Atmospheric Administration (NOAA) on Jan. 12, 2020. NOAA/AP

The Philippine Institute of Volcanology and Seismology raised its danger warning to Level 4 on Sunday, indicating that a “hazardous explosive eruption is possible within hours to days.” Level 5, the highest, means that a hazardous eruption is occurring.

Here’s what to know about the Taal Volcano and what could happen next.

Has the Taal Volcano erupted before?

The Taal Volcano has erupted more than 30 times in the past, according to the Philippine Institute of Volcanology and Seismology.

“It’s not unusual for Taal to erupt,” Ken Hon, a volcanologist at the University of Hawaii at Hilo, tells TIME. “It’s one of the more active explosive volcanoes in the world.”

The Philippines lies on the Pacific “Ring of Fire,” which is the most seismically and volcanically active zone in the world.

But Hon says that because the volcano erupts frequently, eruptions don’t tend to cause catastrophic damage.

“To be more active means that less magma is stored up so you tend to have small to moderate sized eruptions more frequently, rather than storing up the magma and having a larger eruption,” he says.

But he cautions that any eruptions can still be very dangerous in the area around the volcano, and that it’s difficult to say whether—or when—a more powerful eruption could occur. “These things are not completely, 100% predictable,” he says.

Its last major eruption, in 1965, killed hundreds of people, according to the Associated Press.

Lincoln Olayta, a research specialist at the Philippine Institute of Volcanology and Seismology tells TIME that eruptions in 1911 and 1754 are the most notable in the volcano’s recoded history, and may serve as a warning for what the volcano is capable of.

Both of those eruptions were “very violent,” Olayta says. The eruption in 1911 was “very explosive;” it killed more than 1,300 people and lasted for three days, according to Olayta.

The 1754 eruption lasted for almost seven months, and was the volcano’s biggest eruption in recorded history. It buried four towns in Batangas province under ash, volcanic rocks and water, and ashfall as thick as 40 inches was reported in some areas, according to news site Rappler.

Ash poses health risks to millions

Clouds of ash have shrouded nearby towns and Manila, prompting worried residents to stock up on face masks.

Hon tells TIME that residents might have reason for concern due to the type of eruption happening at Taal Volcano, which sits in a lake. He says that the combination of magma with lake water might make the ash particularly harmful to people’s health.

“You don’t want to be breathing these small, little microfragments of glass,” he says. Volcanic ash is typically a mixture of volcanic glass, minerals or crystals and other rock fragments expelled during the eruption.

Motorists drive through a road covered in volcanic ash from Taal Volcano's eruption in Lemery, Batangas province, Philippines on Jan. 13, 2020. Ezra Acayan—Getty Images

“The right amount of water mixed in with that hot stuff causes it to fragment like crazy, so these are small to medium size eruptions that happen here, that’s been the typical M.O. of this volcano, but because of the high degree of fragmentation they are a little bit more dangerous for health,” Hon says.

Authorities in Manila announced on Monday that they planed to crack down on vendors who hiked up the prices face masks as ash engulfed the capital, according to Rappler.

What precautions are being taken?

Philippine officials are calling for evacuations of Taal Volcano Island and areas within an approximately 9-mile radius of the volcano.

As of Tuesday morning, more than 18,000 people were being sheltered temporarily at 118 evacuation centers, according to the National Disaster Risk Reduction and Management Council. Authorities say that more than 30,000 people have been impacted by the eruption of Taal Volcano so far.

Olayta, the Philippine volcanologist, cautions that if the volcano institute raises to alert Level 5, even those who live outside the 9-mile exclusion zone might want to consider evacuation too.

“It will be dependent on how strong, how explosive, how voluminous the eruption is,” he says.

The U.S. Embassy in the Philippines called on citizens to “exercise caution” if they planned to travel in the area near the volcano.

What might happen next?

Scientists say they are monitoring activity closely to determine if a bigger eruption might occur.

“If there is a volcanic tremor—a type of volcanic earthquake — that could signal a possible incoming explosive eruption, so we’re currently observing for that,” Olayta says.

Olayta says that although they are watching volcanic activity closely, it’s still difficult to predict what might happen.

“We can’t say yet if it is going to have a hazardous, explosive eruption or die down, as of now,” Olayta says. “There’s a possibility that the volcano’s activity will die down, which we hope it will.”

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Write to Amy Gunia at amy.gunia@time.com.