Scientists from the University of Utah have determined that Mount Rainier, one of the most prominent peaks in North America, will erupt again. The question of when remains unanswered, but science has recently discovered how: By measuring how quickly Earth conducts electricity ans seismic waves, they’ve effectively “mapped” Rainier’s magma “plumbing.”

“This is the most direct image yet capturing the melting process that feeds magma into a crustal reservoir that eventually is tapped for eruptions,” says geophysicist Phil Wannamaker, of the university’s Energy & Geoscience Institute and Department of Civil and Environmental Engineering. “But it does not provide any information on the timing of future eruptions from Mount Rainier or other Cascade Range volcanoes.”

The interesting part? Some, if not most, of Rainier’s magma reservoir is located not under the volcano, but somewhere between 6 and 10 miles northwest of it. It’s buried about five miles beneath the surface, and “appears to be 5 to 10 miles thick, and 5 to 10 miles wide in east-west extent,” says Wannamaker. They aren’t sure of the north-south extent, due to the nature of the measurement. In fact, plenty of magma could be sitting beneath Rainier, but the 80 electrical sensors used for the experiment were placed in a 190-mile-long, west-to-east line about 12 miles north of the volcano.

About 30% of the reservoir is believed to be molten, with the rest in a sponge-like state. The image shows that the reservoir is the result of a seafloor crustal tectonic plate “subducting” under its neighbor to the east. The resulting water and molten rock then makes its way upward, towards Rainier.

The volcanic flows powering Rainier are about 36 million years old. The U.S. Geological Survey notes that Rainier “is an active volcano that will erupt again.”