The U.S. Geological Survey said on Wednesday that the reservoir of magma beneath Mount St. Helens, a Washington volcano that killed 57 people when it erupted in 1980, is "slowly re-pressurizing." 

The USGS said that it had been monitoring the volcano since 2008, at the end of a four-year long steam eruption. In a statement, the organization said that the build-up of magma is normal for an active volcano, and that it doesn't mean another eruption is imminent:

Analysis of current behavior at Mount St. Helens indicates that the volcano remains active and is showing signs of long-term uplift and earthquake activity, but there are no signs of impending eruption.

Per the statement, scientists from USGS 's Cascades Volcano Observatory and the Pacific Northwest Seismic Network have "been monitoring subtle inflation of the ground surface and minor earthquake activity reminiscent of that seen in the years following the 1980-1986 eruptions." That research has allowed them to conclude with confidence that the magma is re-pressurizing, a scenario hypothesized but previously unconfirmed by scientists. 

That pool of magma lies four to six miles below the surface of the volcano, which measures 8,363 feet in height. 

A time-lapse photo shows how Mount St. Helens changed from 2004-2008, during the prolonged dome-building eruption: 

According to a USGS spokeswoman, the volcano has only grown by about the length of a thumbnail in that period, and might not erupt for another 100 years.