In March 1980, Mount St. Helens, a dormant volcano in the Pacific Northwest, stirred to life. Increasing numbers of earthquakes under the volcano and steam explosions near the summit indicated that this beautiful, symmetrical mountain was getting ready to erupt.

As the signals of an eruption became obvious, officials had to consider which locations around the volcano might be in danger from lava, ash, and mudflows. They needed to define an evacuation area and move all the people within that area to a safe distance from the volcano.