Mount St. Helens is an active stratovolcano in Skamania County, Washington, in the Pacific Northwest region of North America. It is located 96 miles (154 km) south of Seattle and 53 miles (85 km) northeast of Portland, Oregon. The mountain is part of the Cascade Range and was initially known as Louwala-Clough which means "smoking or fire mountain" in the language of the local native Americans, the Klickitats. It was named for British diplomat Lord St Helens who was a friend of George Vancouver, an explorer who made a survey of the area in the late 18th century. This volcano is well known for its ash explosions and pyroclastic flows.

It is most famous for the catastrophic eruption on May 18, 1980 at 8:32am. That eruption was the most deadly and economically destructive volcanic eruption in the history of the United States. It was not, however, the largest eruption in US history. This honor goes to the eruption of Mount Katmai, in 1912, in Alaska, which also ranks, worldwide, as the largest volcanic event of the 20th century (the eruption of Mount Pinatubo was the second largest event).

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