The discovery is particularly important because approximately 2 million people live within 13 miles of the volcano, which is also a popular tourist destination. Understanding its underground activity could help scientists recognize warning signs of future eruptions and better protect nearby communities.

Using six years of satellite observations, researchers tracked subtle changes in the ground around Masaya. They discovered that one part of the volcano initially sank before beginning to rise, possibly indicating that fresh magma was entering a deep reservoir. Meanwhile, the ground around its active Santiago crater and lava lake continued to sink, suggesting that a separate, shallower magma reservoir was behaving differently.

The findings, published in Geophysical Research Letters, reveal that the volcano's underground magma system may be considerably more complicated than previously understood.

"These observations help scientists better understand how magma moves beneath Masaya and may improve monitoring and eruption forecasting in the future," said lead author Lizzie Johnson, who earned her bachelor's degree in geosciences from Penn State in May.

Why Masaya Volcano Is Closely Monitored

Located near Managua, Nicaragua's capital, Masaya is a geologically complex volcano featuring multiple overlapping craters and calderas. Calderas are large depressions that generally form when the ground collapses after magma is removed from an underground chamber.

Although scientists have studied Masaya for years, the movement of magma through its underground reservoirs and passageways remains poorly understood. These systems can change over time, making it difficult to determine exactly what is happening beneath the volcano.

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

Sulfur dioxide is a potentially harmful volcanic gas released as magma approaches or remains near the surface. Even without a major eruption, elevated concentrations can create dangerous conditions for visitors and surrounding communities.

Young Cheol Kim, who completed his doctorate in geosciences at Penn State this summer, explained that monitoring the volcano continuously is essential for distinguishing normal activity from changes that might signal increasing volcanic hazards.

"Doing continuous monitoring allows us to understand the baseline activity and identify see if things are changing," he said, explaining that certain shifts in volcanic behavior could indicate a period of heightened activity or a possible eruption. "And then if there is a need to warn people, we can do that in a timely manner."

Satellites Reveal Hidden Movements Beneath the Volcano

To investigate Masaya's underground activity, the researchers analyzed measurements collected by the Sentinel-1 satellite, which observes the volcano approximately once every 12 days.

The team used a technique called interferometric synthetic aperture radar (InSAR), which can detect extremely small changes in the elevation of Earth's surface. By comparing satellite measurements taken at different times, scientists can identify areas where the ground is rising or sinking.

The technique combines two important capabilities. Synthetic aperture radar sends microwave signals toward Earth's surface, allowing satellites to collect measurements even when clouds obscure the ground. Interferometry compares the returning radar signals to detect changes in distance between the satellite and the surface.

This is especially valuable in tropical regions, where frequent cloud cover can make conventional satellite imagery less reliable.

"Cloud penetration is important for volcanoes because many of them, like Masaya, are in tropical areas where there is often cloud cover," Kim explained. "And when we look at all the different satellite images across multiple years, we can see how the distance between the satellite and the ground changes, where it is uplifting or subsiding."

By examining these subtle movements over time, researchers can investigate processes happening far below the surface without directly observing the magma itself.

Two Magma Reservoirs Behaving in Different Ways

The satellite measurements revealed an intriguing pattern between 2018 and 2024. During the first part of the study, the ground within Masaya's caldera gradually subsided, meaning it moved downward. Later, the same general area began to rise.

Scientists believe this transition could reflect changes in the amount of magma stored underground. When magma drains away from a reservoir, the overlying ground can sink. When additional magma enters and increases pressure within a reservoir, the surface may rise.

The most surprising finding was that the area surrounding Santiago crater did not follow the same pattern. Instead, it continued to sink throughout the six-year study, even as the broader caldera began rising.

"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 said. "At the same time, the area directly around the active Santiago crater deflated over the entire study period independently of the transition to inflation of the main reservoir. This indicates that the plumbing system is more complex and could be due to decreasing magma in a second, shallower reservoir."

Together, these observations suggest that Masaya may have at least two distinct parts of its magma storage system undergoing different changes. A deeper source could be supplying additional magma to the main reservoir, while a shallower reservoir near Santiago crater may be losing magma.

This would help explain why different sections of the same volcano moved in opposite directions.

What the Discovery Means for Future Eruptions

Understanding these underground movements is important because changes in the ground can provide clues about where magma is accumulating, where it is moving, and how it might eventually reach the surface.

The researchers emphasized that their observations did not reveal any immediate danger to communities surrounding Masaya. However, identifying the volcano's normal behavior and understanding its magma supply system could make it easier to recognize unusual changes in the future.

Continued satellite observations, combined with more detailed computer models, could also help scientists better understand the connections between underground magma reservoirs and volcanic activity at the surface.

"Further long-term geodetic and modeling studies will provide a more thorough understanding and allow for effective hazard monitoring and mitigation at Masaya and other lava-lake hosting basaltic volcanoes," Johnson said.

The findings may therefore have implications beyond Nicaragua, particularly for other active volcanoes that contain persistent lava lakes and complex underground magma systems.

From Undergraduate Research to Volcano Science

Johnson became involved in the project after attending a presentation by Christelle Wauthier, a Penn State professor of geosciences and computational sciences hub director at the university's Institute for Computational and Data Sciences.

She completed the research as part of her Schreyer's Honors College thesis, working alongside graduate researchers, including Kim, to analyze satellite measurements and develop her scientific skills.

"I appreciated learning how a research group works," said Johnson, who has recently started a doctoral program in Earth sciences at the University of Hawaii at Manoa, where she will continue studying volcanoes using seismic waves to image below the surface of Kilauea volcano. "I had never had any experience with research, so it was great to be able to join in the group meetings and get to hear about everybody's work. And learning coding was also useful as I continue with my Ph.D."

The research received support from the U.S. National Science Foundation through awards 1945417 and 2523375, along with computational resources provided by Penn State's Institute for Computational and Data Sciences. The authors are solely responsible for the findings and conclusions, which do not necessarily represent the views of the funding organizations.