Although it has been over a quarter-century since the massive volcanic eruption of Mount Saint Helens on May 18, 1980, the mountain (or what's left of it) remains one of the best-known and most-climbed landmarks of the northwest.
Prior to 1980, Mount Saint Helens was 9,677 ft/2950 m high. This made for an interesting arrangement of the peaks in Washington: the five highest were all volcanoes, and the lowest volcano (Saint Helens) was just higher than Bonanza Peak (9511 ft/2899 m), the highest non-volcanic peak in the state. The eruption, however, ruined this nice dichotomy by blowing the top 1344 feet of the mountain, plus a good chunk of its core and north side, out into the sky as ash. Now Saint Helens is only 8333 feet high, dropping it from #5 to about #89 in elevation for Washington State.
Saint Helens is a relatively easy climb. There is a strict quota system that limits the number of hikers, and except in winter you must make reservations to get your permit. Before setting out for this peak, it is important to check the regulations with the National Forest Service. The ascent is best made in spring, since at high elevations the mountain is a giant heap of ash and loose rock that makes for miserable hiking unless it is snow-covered. Besides, the gentle slopes make an awesome backcountry ski run.
From 2004 to 2007, the mountain came to life with minor eruptions, building a new area of lava in the crater, between the "Lava Dome" and the tall crater walls. At the peak rate this lava was flowing, it would have taken about twelve years before St.Helens would have regained its old height. But this minor mountain-building episode is now pretty much over, and the peak has reopened to climbers.
Notes on Summit Elevation and Location
Thanks to Eric Noel for pointing out that there is a new USGS map that supersedes the old one surveyed in 1982/1983, just a couple of years after the eruption. The new map shows a high point of 8333 feet, 32 feet lower that the standard elevation of 8365 from the old map. A recent trip to the crater rim with GPS devices shows that old summit location is now out over the crater, likely having eroded away after the 1983 map was made. The new map agrees exactly with what the GPS showed, in both location and elevation of the new summit.
The popular climbing routes end on the crater rim about a quarter mile east of the true summit, and only a minority of climbers trek over to the true summit from the 8284' spot where the crowds congregate. Anyone who spends time on the rim in the spring needs to take care near the huge snowy cornices that overhang the steep cliffs down into the crater--it is very smart to stay back from the edge, and longstanding climbing tradition is that you don't have to venture on to a cornice to have claimed the summit.
Peakbagger.com now has a peak entry for the Pre-eruption Mount Saint Helens, which can be logged as a separate ascent for those fortunate to have climbed it prior to 1980.