With no explosion in over a century, it might appear that California’s Lassen Peak hasn’t changed much. However, recent findings show the volcano has been slowly sinking for the past three decades, but volcanologists can’t figure out why.

In a recent study, researchers used multiple sets of data from an Interferometric Synthetic Aperture Radar (InSAR) to look at the more than 10,000-foot high volcano located in the Shasta Cascade region of Northern California and track characteristics of deformation in the peak’s vicinity between 1992 and 2010. They discovered that there was a broad range of sinking measuring about 18 to 24 miles and progressing at the rate of about a centimeter per year.

InSAR uses radar images of earth’s surface to track changes on the ground, according to the U.S. Geological Survey. Unlike visible or infrared light, radar waves are able to see through clouds and are equally effective in the dark, allowing researchers to see changes at night and during bouts of bad weather, which comes in handy during a volcanic crisis. The images collected by the radar are compared to each other to determine if there was any movement of the ground surface.

The scientists’ research lead them to believe that this slow sinkage of Lassen Peak has been going on since the 1980s. They believe the source of the sinking is a point located about 5 miles beneath the volcano’s center.

“Time-series analysis suggests that the rate of volume change of this source may have varied over time,” the researchers wrote. “The source geometry and the temporal evolution of deformation contrasts to subsidence observed at nearby Medicine Lake Volcano since the 1950s.”

Denison University assistant professor of geosciences Erik Klemetti speculates there are a few reasons why the volcano is descending.

The most likely culprit is cooling and crystallizing of magma after the 1914-17 eruptions of Lassen Peak,” Klemetti wrote for WIRED. “As magma cools, it loses volume, so any new magma that triggers the eruption over 100 years ago may be slowly losing volume.”

However, this can’t be the only factor contributing to the peak’s shrinking. Klemetti suggests that a change in the flow of water heated by magma underneath the volcano could be to blame as well.

“There is some loose correlation between the times of greater subsidence between 2004-07 and more earthquakes within the area of the hydrothermal system, so there could be a connection there,” wrote Klemetti. “There is even the chance that the M7.3 Landers earthquake in 1992, centered about 520 miles away might have started the ball rolling, as that earthquake seems to have triggered a M3.5 earthquake at Lassen within 13 minutes. However, these events are still correlations rather than causations without further study.”

While there are many possible explanations for the sinking of Lassen Peak, the researchers are relying on their measurements to tell them more.

“We evaluate possible causes of subsidence at Lassen Volcanic Center in light of tectonic setting and hydrothermal activity, and suggest that regional GPS measurements will be key to understanding the role of crustal extension plus other hydrothermal/magmatic processes in deformation during recent decades,” they wrote.