Beneath one of Central America’s most active volcanoes, two reservoirs of molten rock appear to be moving in opposite directions. While one is filling with magma, the other continues to lose pressure, exposing a hidden complexity that could make it harder for scientists to anticipate when the volcano might erupt, endangering millions living nearby.
The discovery comes from a six-year satellite investigation of Nicaragua’s Masaya volcano, located just 12 miles southeast of the capital, Managua, where roughly 2 million people live within 12 miles of the volcanic complex.
Researchers at Pennsylvania State University found that a large magma reservoir approximately two miles beneath Masaya began expanding around mid-2022, likely indicating a renewed supply of molten rock rising from deeper inside Earth.
However, a second reservoir situated just 650 feet beneath the volcano’s active Santiago crater continued contracting throughout the observation period.
The findings, published in Geophysical Research Letters, suggest Masaya’s underground magma system is more complicated than formerly recognized, with important consequences for monitoring volcanic activity as well as anticipating potentially dangerous eruptions.
“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,” Penn State graduate and lead author Lizzie Johnson said in a press release. “Although Masaya last produced a major lava flow in 1772, its persistent degassing often produces dangerously high concentrations of sulfur dioxide volcanic gas.”
A Volcano That Changed Direction
Masaya has a long history of volcanic events, including powerful prehistoric explosive eruptions and a major lava flow in the late 1700’s.
More recently, the volcano has become known for persistent sulfur dioxide emissions and an intermittent lava lake within Santiago crater, which has attracted visitors to the surrounding national park.
To investigate what was happening underground, researchers analyzed radar observations collected by the European Space Agency’s Sentinel-1 satellites between January 2018 and February 2024.
Using a technique called interferometric synthetic aperture radar, or InSAR, the team measured extremely small changes in the distance between the satellites and the volcano’s surface.
These measurements uncovered that the central portion of Masaya’s caldera had moved approximately 1.2 inches away from the satellites between 2018 and mid-2022, consistent with a gradual loss of pressure within an underground magma reservoir.
Around mid-2022, however, that pattern reversed. The ground began moving toward the satellites, indicating that the deeper reservoir was expanding again.
“When it’s subsiding, that means that there’s probably magma draining away from the volcano, but once the ground starts uplifting again, that can mean that there’s potentially new magma moving in,” Johnson explained.
Computer modeling placed the expanding reservoir approximately 10,500 feet beneath the caldera. The researchers estimated it had lost about 1.3 million cubic meters between 2018 and 2022, then gained roughly 450,000 cubic meters through early 2024.
But the satellite observations also revealed something unexpected.
Two Magma Reservoirs, Two Different Behaviors
Despite the apparent replenishment of the deeper reservoir, the ground immediately surrounding the Santiago crater continued showing signs of deflation. Modeling suggested this specific deformation originated from a second magma reservoir approximately 650 feet underground.
The shallow reservoir lost an estimated 141,000 cubic feet during the first observation period, followed by another 212,000 cubic feet during the second.
The opposing trends suggest that renewed magma accumulation deeper underground does not necessarily translate into an immediate increase in pressure beneath the active crater.
Santiago crater’s lava lake and persistent volcanic gas emissions are closely linked to this shallow magma system, making changes in its underground activity particularly important to comprehend the volcano’s behavior.
The researchers suggest the ongoing deflation near the crater could reflect less magma entering its shallow underground pathways, consistent with the decline in lava lake activity observed after 2016.
Masaya’s two-reservoir arrangement also resembles the magma plumbing system beneath Hawaii’s Kīlauea volcano, where shallow magma storage interacts alongside a deeper reservoir.
Understanding how magma moves between these levels could help researchers distinguish normal fluctuations in volcanic behavior from changes that precede an eruption.
Could Masaya Be Preparing for Another Eruption?
Perhaps the most consequential finding involves what happened after the first six-year observation period.
In an extended analysis covering 2024 through April 2026, the researchers estimated an additional volume increase of approximately 56.5 million cubic feet within Masaya’s deeper magma system.
Combined with the earlier inflation, that represents roughly 72 million cubic feet of modeled volume increase since 2022.
Nevertheless, the researchers caution that considerably more inflation may be necessary before renewed lava activity occurs.
Previous observations linked the appearance of Masaya’s lava lake to about 141 million cubic feet of cumulative reservoir volume change. However, that historical comparison does not establish a reliable threshold for future eruptions, and the new findings do not suggest that an eruption is imminent.
The concern is understanding when the volcano’s behavior changes enough to warrant closer attention.
For the millions of people living near Masaya, the discovery highlights why monitoring a volcano’s visible crater alone may not reveal the full extent of what is happening underground.
“Doing continuous monitoring allows us to understand the baseline activity and identify if things are changing,” study co-author Dr. Young Cheol Kim said. “And then if there is a need to warn people, we can do that in a timely manner.”
The study, “Two active magma reservoirs identified at Masaya Volcano, Nicaragua, using satellite geodesy,” appeared in Geophysical Research Letters.
Tim McMillan is a retired law enforcement executive, investigative reporter and co-founder of The Debrief. His writing typically focuses on defense, national security, the Intelligence Community and topics related to psychology. You can follow Tim on Twitter: @LtTimMcMillan. Tim can be reached by email: tim@thedebrief.org or through encrypted email: LtTimMcMillan@protonmail.com