Scientists are warning that the Mount St. Helens volcano in the northwestern United States could erupt as early as Sunday.
Geologists say there are signs that magma is moving underground and building pressure inside the volcano.
The government has raised its alert level for Mount St. Helens and told people to leave the immediate area. Saturday, hundreds of visitors were evacuated from an observatory eight kilometers away.
However, officials say residents of the nearest town, 48 kilometers to the west, are probably safe.
Scientists expect any new eruption to be much smaller than the 1980 Mount St. Helens explosion that killed 57 people and flattened thousands of hectares of forest.