Mount Rainier - one of many volcanoes found in the Cascade Range - is reflecting in the Reflection ... [+] Lakes in Mount Rainier National Park in Washington State, USA.LightRocket via Getty ImagesA seismic survey challenges the long-standing belief that only active volcanoes have large magma bodies sitting beneath them.A research team used seismic waves to characterize magma chambers beneath the surface of six volcanoes of various sizes and dormancy within the Cascade Range. They found that all of the volcanoes, including dormant ones, have persistent and large magma bodies.Their results are surprising given that some of these volcanoes, such as the Crater Lake volcano in Oregon, have not been active in millennia."Regardless of eruption frequency, we see large magma bodies beneath many volcanoes," explains Guanning Pang, postdoctoral researcher at New Cornell University and first author of the study. "It appears that these magma bodies exist beneath volcanoes over their whole lifetime, not just during an active state."The fact that more volcanoes have sustained magma bodies is an important consideration for how researchers may monitor and predict future volcanic activity.We used to think that if we found a large amount of magma, that meant increased likelihood of eruption, but now we are shifting perception that this is the baseline situation, so Pang.The results suggest that an eruption does not completely drain a magma chamber, instead, it lets off some of the excess volume and pressure. The chamber is then refilled over time reaching a stable phase. The volcano appears calm on the surface with no visible activity on the surface.MORE FOR YOUMore than 800 million people are living within 60 miles (100 kilometers) of a volcano. In the last 500 years more than 278,000 people were killed during an eruption, so optimizing monitoring efforts is of highest priority.The U.S. Geological Survey has been expanding and upgrading its volcanic monitoring networks in the Cascade Range and elsewhere as part of the National Volcano Early Warning System, with the aim of detecting signals of an impending eruption as early as possible. Plans are already in the works to expand the new magma monitoring system and see if the Cascade discovery translates to other locations, including Alaska.The study, "Long-lived partial melt beneath Cascade Range volcanoes," was published in the journal Nature Geoscience and can be found here.Additional material and interviews provided by Cornell University.