When Spanish conquistadors first laid eyes on the Masaya volcano’s lava lake in the early 1520s, some believed it must be the entrance to hell. Recent activity from Nicaragua’s busiest volcano has mostly consisted of passive degassing, lava lake formation, and vent-clearing blasts, but Masaya has also produced at least four massively explosive Vesuvian eruptions throughout its history.

That’s unusual for a basaltic shield volcano like Masaya. These volcanoes tend to produce slow, effusive lava flows, but Masaya can transition between effusive eruptions and more dangerous explosive behavior. Today, understanding what drives those transitions is critical to assessing the threat the volcano poses to millions living within 13 miles (21 kilometers) of it.

To that end, researchers recently used satellite data to investigate how the ground around Masaya has changed over a six-year study period. Their findings, published in the journal Geophysical Research Letters, suggest this volcano is fed by not one, but two active magma reservoirs.

“Having a thorough understanding of Masaya’s volcanic activity and its related hazards is critical, as around 2 million people live near Masaya, and the caldera is a national park and popular tourist destination,” Lizzie Johnson, who led the research as an undergraduate in Penn State’s geosciences program and is now pursuing her PhD in Earth sciences at the University of Hawaii at Manoa, said in a statement.

“Although Masaya last produced a major lava flow in 1772, its persistent degassing often produces dangerously high concentrations of sulfur dioxide volcanic gas,” Johnson added.

Masaya Volcano is a complex caldera system that includes a nested series of cones and craters with an active vent in the Santiago crater.

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