Romania’s Ciomadul volcano last erupted some 30,000 years ago, but as a new study published in Earth and Planetary Science Letters reports, between 5 to 14 cubic miles of magma still simmer below the purportedly “extinct” peak.

These findings don’t necessarily portend Ciomadul’s imminent eruption. Instead, as study coauthor Mickael Laumonier of France’s Université Clermont Auvergne tells National Geographic’s Maya Wei-Haas, the researchers’ work emphasizes the importance of assessing both active and inactive volcanoes. (Per Encyclopedia Brittanica’s Nora Gonzalez, volcanoes are considered active if they have erupted in the past 10,000 years, dormant if they have the potential to erupt again, and extinct if they have not erupted in more than 10,000 years and are unlikely to erupt again.)

“We look preferentially at active volcanoes—obviously because they show evidence of real risk,” Laumonier says. “But we shouldn’t forget other relatively recent young volcanoes, because they could present a risk that we should assess.”

According to the paper, Laumonier and colleagues from Switzerland, Hungary and Romania drew on geophysical and geochemical analysis, as well as numerical simulations of Ciomadul’s thermal evolution, to estimate how much magma is stored beneath the volcano. The results, Wei-Haas writes, suggest Ciomadul could be concealing magma of a maximum volume greater than the space taken up by 20,000 Great Pyramids of Giza.

It’s worth noting, however, that the scientists are “still not ... saying that’s what the case is,” as Janine Krippner, a volcanologist at the Smithsonian Global Volcanism Program who was not involved in the study, points out. “It’s saying, We have a lot of data that show this is what it might be.”

The new study builds on previous research pointing toward the existence of a hidden magma reservoir below Ciomadul. In February 2018, an article in the Journal of Volcanology and Geothermal Research proposed using the term “volcanoes with Potentially Active Magma Storage” to demonstrate long-dormant systems’ “potential [for] rejuvenation,” while a 2010 paper published in the journal Radiocarbon called for “further detailed studies … on this seemingly inactive volcano in order to evaluate the possible renewal of volcanic activity in the future.”

Per National Geographic, Laumonier’s team found that the rocks in the upper crust below Ciomadul are, on average, 15 percent molten. Some areas are as high as 45 percent molten—crucially, this number represents the tipping point at which researchers believe eruptions can occur, says Michael Ackerson, curator of rock and ores at the Smithsonian’s National Museum of Natural History. Below this level, he adds, volcanic systems are “all locked up by crystals” and cannot erupt.

As Ackerson, who was not involved in the new research, explains, “The traditional trope of a magma chamber is this big, gigantic menacing-looking, red-hot blob of magma sitting in the crust that’s about to erupt and kill us all.”

In actuality, Wei-Haas writes, magma reservoirs “spend most of their lives quietly stewing in the crust, … forming a mushy, stony soup with varying proportions of crystals [that] melt throughout the system.” The exact mechanics of this phenomenon, including how it varies from one volcano to the next and what it means for classifying active versus extinct systems, remain unclear.

Separately, a long-dormant volcano nestled on Russia’s Kamchatka Peninsula has made headlines in recent weeks due to an apparent increase in seismic activity. According to a study published in the Journal of Volcanology and Geothermal Research, tracking stations installed near the Bolshaya Udina volcano recorded 2,400 seismic events between October 2017 and February 2019. Carolyn Gramling of Science News reports that the findings have divided researchers: Some view the rumbles as a sign of the volcano’s imminent eruption, while others attribute the seismic activity to actively erupting volcanoes in the surrounding region.

Regardless of whether Bolshaya Udina and Ciomadul actually end up erupting, Laumonier and his colleagues warn, “Some [volcanoes] reawaken, posing a particular threat because little is known about the way they endure and stir back to life.”

“That a seemingly dead volcano like Ciomadul is actually capable of erupting in the future calls for renewed attention to inactive’ volcanoes worldwide and perhaps for a redefinition of their activity/inactivity status,” the scientists conclude.