A New Zealand geologist has triggered concerns about the possibility of another American "supervolcano" building under Mt St Helens.
GNS scientist Graham Hill has found big, connected channels of semi-molten rock beneath the southern Washington state mountain, which killed 57 people when it erupted in 1980, New Scientist magazine reports.
The discovery has revealed what may be an extraordinarily large zone of semi-molten rock, which would be capable of feeding a giant eruption.
The New Zealander led a team that set up sensors around Mt St Helens and found a column of conductive material that extends downwards from the volcano.
About 15km below the surface, the relatively narrow column appears to connect to a huge zone of conductive material.
This larger zone was first identified in the 1980s by another survey, and was found to extend all the way to beneath Mt Rainier 70km to the northeast, and Mt Adams 50km to the east.
It was thought to be a zone of wet sediment, water being a good electrical conductor.
But because the new measurements showed an apparent conduit connecting this conductive zone to Mt St Helens, Dr Hill now thought the conductive material was more likely to be a semi-molten mixture.
Its conductivity was not high enough for it to be pure magma, so it was more likely to be a mixture of solid and molten rock, he said.
But Oregon State University expert Gary Egbert, who was not a member of the research team, told the Oregon Public Broadcasting Service that he would be more cautious.
"It seems likely that there's some partial melt down there," given that it is a volcanic area, he says.
"But part of the conductivity is probably just water."
If the structure beneath the three volcanoes is a vast bubble of partially molten rock, it would be comparable in size to the biggest magma chambers ever discovered, such as the one below Yellowstone National Park.
Every few hundred thousand years, such chambers can erupt as so-called supervolcanoes - the one below Yellowstone did about 640,000 yearsago.
These enormous eruptions can spew enough sunlight-blocking ash into the atmosphere to cool the global climate by several degrees celsius.
Asked whether Mt St Helens could erupt like this, Dr Hill said: "A really big, big eruption is possible if it is one of those big systems like Yellowstone.
"I don't think it will be tomorrow, but I couldn't try to predict when it would happen."
He said further measurements probing the structure of the crust beneath the other volcanoes in the area could help to determine if the zone connects to them all.