Why it matters: For centuries, people lived in the shadow of Mount St. Helens, but the threat of an eruption had always been theoretical until May 18, 1980.

The eruption produced a massive ash cloud that circled the Earth for 15 days. While volcanic eruptions can have a long-lasting impact on Earth’s temperature, Mount St. Helens’ eruption didn’t.

In 2004, for example, researchers had about a week’s notice before a minor eruption at Mount St. Helens. In 1980, there was only one instrument on the mountain.

Retired Col. Terry Connell, left, former U.S. Army Corps of Engineers Portland district commander during the 1980 Mount St. Helens eruption, and Steve Stockton, right, the retired director of ...

He said in the 90s, he even hiked up the volcano, and had the opportunity to look down into the crater. RELATED: Mount St. Helens eruption: See rare footage of the 1980 eruption from KGW archives ...

ST HELENS, Wash — Engineers who spent the days and years following the 1980 eruption of Mount St. Helens working on the recovery of the region, returned Tuesday to check in on some of their work.