From the Mount St Helens Wikipedia page:
Mount St. Helens is most notorious for its major 1980 eruption, the deadliest and most economically destructive volcanic event in the history of the United States. Fifty-seven people were killed; 250 homes, 47 bridges, 15 miles (24 km) of railways, and 185 miles (298 km) of highway were destroyed. A massive debris avalanche triggered by an earthquake measuring 5.1 on the Richter scale caused an eruption that reduced the elevation of the mountain’s summit from 9,677 ft (2,950 m) to 8,363 ft (2,549 m), leaving a 1 mile (1.6 km) wide horseshoe-shaped crater. The debris avalanche was up to 0.7 cubic miles (2.9 km3) in volume.
Someone just found a set of 34 slides, from photos taken on a plane circling the volcano as it erupted. These never-before-published photos are worth a look.
To me, the most amazing thing about this event is how, in the space of a few days, an almost perfectly conical mountain was practically leveled.