At 8:32 a.m. on May 18th, 1980, a 5.2 magnitude earthquake struck below the north face of Washington's Mount St. Helens.
Vulcanologist David Johnston was camping only a few miles north of the volcano, and radioed the USGS base in Vancouver, Washington: "Vancouver, Vancouver, this is it!"
The earthquake triggered the largest debris avalanche in recorded history, wiping out the northern summit of the volcano and exposing its core.
At 8:39 a.m., Mount St. Helens erupted 60,000 feet into the air. It was the biggest eruption to rock the volcano in 123 years.
"During the eruption, the explosions rocked the mountain and could be heard almost 200 miles away," KOIN-TV correspondent Ken Woo reported for CBS News.
"The major north-south highway, Interstate 5, has been closed north of Portland, Oregon to Seattle, Washington. The closures are because of fear of flooding -- walls of water 30 feet high have been sighted on the Toutle River," Woo said.
Winds drifted 520 million tons of ash across the state, creating oddly dark skies in the middle of the day. Visibility in nearby towns was reduced to almost zero. Even Spokane, Washington, more than 250 miles to the northeast, was darkened.
Johnston did not survive the blast -- he was one of 57 people killed that day. It was the deadliest volcanic eruption in U.S. history, which also devastated the area's wildlife. Almost 7,000 big game animals and 12 million juvenile salmon were lost.
The volcano has experienced a number of much smaller explosions since then, the most significant being five more eruptions throughout the spring, summer, and fall of 1980.
According to the USGS, Mount St. Helens is likely to erupt again "in our lifetimes." However, a landslide and massive eruption like those of May 18, 1980 are unlikely now that a deep crater has formed at the top of the volcano.