A "swarm" of 130 earthquakes have trembled beneath Mount St Helens in the past eight weeks, seismologists have revealed.
The US Geological Survey (USGS) said the series of small quakes showed "the volcano is still very much alive".
Although there are no signs of an imminent eruption, the volcano - 95 miles south of Seattle - is recharging, the seismologists have said.
There are no unusual gases reported, and no signs that the magma - the molten rock beneath the surface of the Earth, is expanding.
Most of the tremors are less than 0.5 in magnitude and the largest was 1.3 - rumbling about 1.2 to four miles beneath the surface.
Although the quakes are relatively small, it is the frequency that is concerning scientists.
"Earthquake rates have been steadily increasing since March, reaching nearly 40 located earthquakes per week," a USGS statement read.
The earthquakes are volcano-tectonic in nature, indicative of a slip on a small fault."
The USGS said the volcano's collection of magma is "re-pressurising", but said the process can continue for years without an eruption.
Similar patterns of "earthquake swarms" were detected in 2013, 2014 and in the 1990s, said the USGS.
Mount St Helens erupted on 18 May 1980, leaving a huge crater and spewing hot ash across the northwestern US, killing 57 people.
It also started forest fires and caused floods as the snow melted from mountain tops.
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