In the 11 days since Hawaii’s Kilauea volcano started erupting, its fury has continued to grow. The shield volcano’s massive stores of molten rock have already forced their way through 17 new fissures, and it’s begun shooting boulders into the sky. Adding to the mounting terror its unpredictability poses, Kilauea has also started to literally scream.

Like the call of a Ringwraith leaving the Eye of Sauron, the screech likely occurred as magma burst through one of the new fissures. As Reuters reported in an updated post on Monday, the two new fissures that opened up on Sunday hurled “burst of rock and magma with an ear-piercing screech.” Scientists have observed this sort of thing, technically known as a “harmonic tremor,” during volcano eruptions before, though they’re not entirely sure where the sound comes from. But they have some ideas.

As scientists at Oregon State University explain, the sounds we hear are ultimately caused by underground gases forcing their way through tiny channels at extremely high pressure. “The high pressures associated with the gas-rich magma within these cracks, pipes and conduits can cause the volume to resonate similar to a pipe organ,” they write. A simple toy whistle is also an apt comparison. Essentially, any highly compressed air forced through a narrow pipe will create a sound; the frequency (which is closely linked to the pitch) of that sound generally increases as the length of the whistle decreases. In the case of a volcano, the “whistle” or “pipe” is the channel through which the magma is trying to push through.

The sound of Alaska's Redoubt Volcano screaming in 2009, as captured by researchers at the University of Washington.

Relatedly, in 2013, scientists at the University of Washington explained in the Journal of Volcanology and Geothermal Research that the high-pitched screech is linked to the sound of rapidly moving earthquakes, which commonly accompany volcanic eruptions. The day after Kilauea started erupting, an earthquake of magnitude 6.9 shook the region, which, according to the paper, is caused by magma forcing its way upward through extremely narrow channels in the Earth where the pressure is extremely high. Every time magma forces its way through a channel, it’s thought to trigger a small earthquake. As magma inches upward, the earthquakes get smaller and smaller as the pressure becomes increasingly high. Eventually, the quakes get so small and happen so quickly that they blend into one continuous, or gliding, tremor.

“Because there’s less time between each earthquake, there’s not enough time to build up enough pressure for a bigger one,” said lead author and postdoctoral fellow with the Volcano Science Center Alicia Hotovec-Ellis, Ph.D., in a statement. “After the frequency glides up to a ridiculously high frequency, it pauses and then it explodes.”

By this logic, the chilling screams will likely stop as the magma stops trying to force itself through new cracks in the ground. Unfortunately, as Hawaiian Volcano Observatory spokesperson Janet Babb said in early May, there’s no knowing when Kilauea will stop. When it exploded in 1955, for example, it didn’t stop erupting for 88 days. The good news is that the piercing screams, however terrifying, are also a handy warning call for people in the area to evacuate immediately.