Mount Saint Helens erupted in a colossal explosion 30 years ago, on May 18, 1980 (incidentally, exactly a week before my little sister was born).
The eruption was so great that the entire northern side of the mountain was blasted off, leaving a huge crater. Several minor earthquakes and venting gases in the months before the eruption meant that many people were studying and observing the mountain, but people were shocked at the size of the eruption. Fifty-seven people died (not to mention hundreds of thousands of trees and untold wildlife), and ash blew as far away as Boston.
Speaking of Boston, the Boston Globe put together an amazing set of photos from the eruption. Be sure to visit the link to see all of them.
Here’s a youtube video of the eruption.
If you have an hour, I’d recommend watching this recent episode of the science show NOVA, which is about the eruption and the state of Mount St. Helens now, thirty years later. Is it getting ready to erupt again?