The volcano erupted violently on May 18, 1980, killing 57 people and altering the landscape for miles around.
“The magma reservoir beneath Mount St. Helens has been slowly re-pressurizing since 2008,” the U.S. Geological Survey said in a statement.
“It is likely that re-pressurization is caused by (the) arrival of a small amount of additional magma 2.5 to 5 miles beneath the surface.”
But seismologist Seth Moran at the Cascades Volcano Observatory told CBS News that the volcano could accumulate pressure inside for a long time before it starts to erupt.
“The reassuring thing is: when it’s really ready to erupt, it gives lots and lots of signs,” Moran told the broadcaster.