US scientists fear a surge in activity at the Mount St Helens volcano in Washington State might be a sign it is on the verge of erupting.
Any possible eruption would likely be small
But they add that any possible volcanic event would be far smaller than the devastating 1980 collapse, which resulted in the deaths of 57 people.
Steam venting and small explosions - reaching a maximum of 5km away - are the likely outcome of present activity.
But seismic activity at the volcano is now at levels not seen since 1986.
The area around Mount St Helens has now been sealed off.
The US Geological Survey Cascades Volcano Observatory (CVO) in Vancouver, Washington, released a notice of "volcanic unrest" on 26 September. Since then, the activity has increased still further.
The eruption nearly 25 years ago sent half a billion tonnes of ash into the air, leaving Portland in Oregon - the nearest major city at a distance of about 80km (50 miles) away - covered in a thick layer of the volcanic debris.
Earthquakes associated with Mount St Helens started occurring more frequently than normal on 23 September and yesterday were reported to be occurring at a rate of three to four a minute - almost double the previous day's count.
These earthquakes were also reported to have increased in magnitude.
In their notice of volcanic unrest, CVO says that changes in the pattern of seismic activity suggest there is an increased likelihood of a hazardous event.
They have been in contact with Washington State and county emergency management officials to discuss hazard and response issues.
Access to the crater rim and to hiking trails around the volcano has been barred.
A dome builds up on the mountain over time from viscous lava which gradually erupts from the volcanic vent.
Small earthquakes beneath the volcano's dome have stirred it
The present dome on Mount St Helens formed between 1980 and 1986 and sits in the crater which was formed by the massive eruption in 1980.
There has been some evidence of movement of the lava dome from GPS measurements but this has not been reinforced by seismic and gas analysis.
The type of earthquakes that are occurring are located 1km beneath the lava dome and are called volcano tectonic earthquakes.
These quakes are not usually indicative of the movement of fluid magma beneath the volcano. The gas measurements that the scientists have taken also agree with this.
Describing the possible outcomes of this recent increase in activity, Jon Major, a geologist at the CVO told BBC News Online: "One possibility is that this period of volcano indigestion will just die down and go away."
"Another possibility is that we get some explosive activity."
This activity could throw up rock fragments that would land in the crater itself or on to the flanks of the volcano.
Dr Major added a third outcome was the possibility that lava would work its way up the volcanic vent and "ooze another lava pancake on to the dome, increasing the dome size".
The scale of the volcano now is nothing like it was before the devastating eruption in 1980. Dr Major emphasised that there was not likely to be a threat to the public at distances greater than 5km from the volcano.
The worst case scenario for distances beyond that is if the heat from the magma melts the glacier in the crater.
"This could cause a flood that could generate a mudflow down the flanks of the volcano, into the North Fork Toutle River," Dr Major added.