Scientists are on high alert as they monitor the Taal volcano, which began erupting Sunday on the Philippine island of Luzon. The volcano is in a heavily populated area just 45 miles from the megacity of Manila and is capable of exploding with devastating effects.

Taal has erupted in prehistoric times with enough violence to leave a crater many miles across and spew enough ash into the stratosphere to temporarily cool the entire planet.

Taal is a volcano within a volcano. The ash cloud and lava fountains are emerging from a crater on Volcano Island, a tourist destination that sits in the waters of Lake Taal. The well-defined escarpments rimming the lake reveal its true nature: It is a giant crater formed by massive eruptions.

Lake Taal is similar to Crater Lake in Oregon and Yellowstone Lake in the heart of Yellowstone National Park. Each marks a caldera formed when an eruption many thousands of years ago caused the ground to collapse into evacuated magma chambers below.

Scientists with the Philippine Institute of Volcanology and Seismology (PHIVOLCS) said Wednesday that the eruption continues but is “generally weaker.” Evacuation orders remain in place, and PHIVOLCS continues to warn that an explosive eruption could occur at any time. On Volcano Island proper, about 8,000 people have evacuated. There are no reports of fatalities.

Scientists have recorded 595 volcanic earthquakes since Sunday, including 30 over a 10-hour stretch Thursday, which “signifies continuous magmatic intrusion beneath the Taal edifice, which may lead to further eruptive activity,” PHIVOLCS reported.

There is no sign that Taal is about to blow in a catastrophic eruption, but the best predictor of a volcano’s future behavior is what it has done in the past, and Taal has erupted more than 30 times in five centuries.

A two-day eruption in 1965 created a “base surge” of horizontally racing, superheated gas and ash that killed 150 people. More than a thousand died in an eruption in 1911. A prehistoric eruption about 3,500 years ago was on the same scale as the 1991 eruption of Mount Pinatubo, which killed hundreds of people, blanketed Luzon in ash, and ejected so much material into the stratosphere that it cooled the planet by about 1 degree Fahrenheit for more than a year.

“Each volcano has its own personality. You know just how much you can tease your sister before she’s going to blow up. This volcano, we do not know very well. Worldwide, we have not seen enough large volcanoes like this erupt catastrophically, so we don’t know what the signs are,” said Tracy Gregg, a geologist at the State University of New York at Buffalo.

“The time scales over which these volcanoes erupt are beyond our human time scales. They’re not predictable,” said U.S. Geological Survey geologist Alexa Van Eaton. “But they do usually give warning signs when they ramp up. That’s why it’s so important to have geologists on the ground . . . who are monitoring, and listening, and sniffing the gases, and doing everything they can to keep tabs on what those signals and signs are telling them.”

As magma rises, the surface of the land rises, too, and that “doming” can be detected by GPS instruments. The moving magma breaks rocks beneath the surface with sufficient violence to generate earthquakes. An earthquake swarm — tremors of magnitude 3 or so — typically precedes and coincides with an eruption, which is when the magma reaches the surface.

These eruptions can take many forms, and they are typically explosive, such as when compressed gases escape the confines of the crust and expand thousands of times, or when molten rock makes contact with water. In a pyroclastic flow, clouds of hot gas and volcanic particles can move horizontally and kill everything in their path. Even when the eruption is over, dangers lurk, because heavy rains can turn the deposited ash into a lahar, a flood of mud and debris. Lahars created by a volcanic eruption in Colombia in 1985 claimed more than 20,000 lives.

The key to forecasting the behavior of volcanoes is monitoring, scientists say. But even in the United States, many of the country’s 161 active volcanoes have minimal instrumentation, according to Charles Mandeville, the head of the Volcano Hazards Program of the U.S. Geological Survey.

“We’ve got bad-actor volcanoes that aren’t optimally instrumented to the level that they should be,” he said. Congress passed legislation last year authorizing the USGS to create a National Volcano Early Warning System, which has been 14 years in the making and now awaits appropriations, Mandeville said.

“Volcanoes are undermonitored,” agrees Michael Manga, a geologist at the University of California at Berkeley who studies volcanoes. “Half the volcanoes in the United States don’t have a single instrument to record what they’re doing. And we’re a wealthy country, so you can imagine the situation globally.”

Volcanoes are geological wild cards. Their eruptions can come in many sizes. A major caldera-forming eruption would be expected somewhere on Earth every 150 years or so, Mandeville said. An example would be the eruption of Krakatau in 1883, the explosion of which could be heard thousands of miles away.

In that kind of event, the magma chamber vents gas and lava in a circular pattern. So much magma shoots skyward in multiple eruptions that the center of the structure collapses into the chamber below. The giant pit gradually fills with water and sediment, but it remains a clear scar created by astonishing volcanism.

“A caldera-forming eruption simply means that when you lift the surface of the earth and you explode the magma out from beneath the surface, then you lose the roof support, and the roof collapses down, in essentially a circular hole. The caldera is the hole left after the subsidence,” said Robert Smith, a geophysicist at the University of Utah who has studied Yellowstone for more than six decades.

The Philippine archipelago is the product of volcanoes, part of the Pacific “Ring of Fire” that includes volcanoes in Japan, Alaska and Chile. The Philippines has more than 20 active volcanoes.

The Taal caldera, as defined by geologists studying the topography of the area, is a bit broader than the lake that formed within it, and sits atop two intersecting faults. Another quirk of Taal: There is a tiny island within Main Crater Lake, which is on Volcano Island — which, of course, is within Lake Taal, which is itself on the island of Luzon.

So it’s an island on a lake on an island on a lake on an island.