In 2014, scientists blasted Mount St. Helens, one of America's most closely watched volcanoes, with 23 explosions, revealing a vast magma system underground

Mount St. Helens has been studied for decades, yet scientists still cannot simply look beneath the volcano and see how magma moves through the Earth. That changed dramatically in 2014, when researchers carried out one of the most ambitious seismic experiments ever attempted at the volcano.

As part of the Imaging Magma Under St. Helens (iMUSH) project, scientists deliberately set off 23 controlled explosions around Mount St. Helens. They then used thousands of seismic instruments to record the waves traveling through the ground.

The goal was to create an underground map of the volcano. What emerged was far more complicated than a simple magma chamber.


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An unusual experiment beneath Mount St. Helens

In late July and early August 2014, a field team of about 70 people worked around the volcano as part of the iMUSH experiment. Instead of waiting for earthquakes to generate seismic waves, researchers created their own signals.

The 23 explosions were placed at different distances from Mount St. Helens. As the resulting waves moved through the Earth, hundreds and then thousands of instruments recorded how quickly they arrived.

That information allowed scientists to determine how seismic waves behaved in different types of rock beneath the volcano.

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The explosions were not designed to simulate an eruption. They were artificial seismic sources that effectively allowed researchers to probe the Earth's interior.

Thousands of instruments recorded the waves

The scale of the operation was remarkable. Researchers deployed roughly 800 to 1,000 seismic recorders along each of two major profiles, with another 1,600 to 1,800 instruments arranged in larger arrays around the volcano.

Closer to the summit, hundreds of additional instruments and about 920 nodal seismic units were positioned along trails within roughly 7.5 kilometres of Mount St. Helens.

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The experiment ultimately captured approximately 80,000 seismic traces from the 23 explosions. That dense network gave scientists the ability to examine the volcano from multiple directions and at different depths.

Scientists found a major magma-storage zone

One of the most important discoveries from studies using the iMUSH data was evidence of an upper-crustal low-velocity zone roughly 4 to 15 kilometres beneath Mount St. Helens. Seismic waves travel more slowly through this region than through the surrounding rock.

That can happen when rocks are hot, fractured or partially molten, making the zone an important clue to where magma is stored.

A 2023 presentation to the Seismological Society of America described this upper-crustal low-velocity zone as the primary magma reservoir, with estimates suggesting it could contain approximately 10% to 12% partial melt.

That does not mean scientists discovered a giant underground lake of liquid magma.

Instead, a magma reservoir is more accurately viewed as a complicated mixture of molten material, crystals and surrounding solid rock.

Mount St. Helens has a complicated underground plumbing system

The seismic images also suggested that magma beneath Mount St. Helens does not simply sit inside one enormous chamber connected to the surface by a narrow pipe.

Instead, the volcano appears to have a much more complicated plumbing system extending through the crust.

Later studies using iMUSH data identified variations in seismic properties at different depths, including unusual high-velocity regions deeper underground.

Scientists have considered several possible explanations for these structures, including accumulated crystallised magma known as magmatic cumulates.

Researchers also examined seismic-wave properties, including the relationship between P-wave and S-wave velocities, to better understand the composition and structure of the crust.

Why the discovery matters

Mount St. Helens is one of America's most closely watched volcanoes, partly because of its devastating 1980 eruption.

Understanding where magma is stored and how it travels beneath an active volcano could provide important clues about volcanic behaviour.

The 2014 experiment offered something rare: a detailed look beneath the volcano using signals scientists created themselves.

Rather than revealing one simple underground chamber, the research exposed evidence of a complex magmatic plumbing system, including a major magma-storage zone about 4 to 15 kilometres below Mount St. Helens.

For volcanologists, the hidden world beneath the mountain is now a little less mysterious — but the picture emerging from the data suggests there is still much more to discover.