Even before May 18, 1980 when Mt. St. Helens blew its top sky-high while vaporizing a vast area to its north, it was the smallest of Washington's 5 volcanoes at 9677 ft (2950 m). The blast removed far more than that 1312 feet from the height of the mountain. It left a mere shell of what had been a beautifully symmetrical cone. The melting of several glaciers contributed to the massive floods. Sliding earth buried wonderful stands of virgin forest and recreational facilities and killed untold wildlife. A beautiful tree-lined lake was dammed to a higher level and filled with heat-stripped logs. Condensed rock vapor was carried on the wind to be deposited up to thousands of miles away.
For seven years recreational climbing was banned on Mt. St. Helens. Then in May 1987 it was reopened. Only the south routes are still allowed. Access to the crater is strictly forbidden.
On clear mornings I can see the mountain as I drive out of my neighborhood to work. That view is a constant reminder of the power of nature. I have always regretted that I had moved from Vancouver, WA to Juneau, AK only three months before the eruption. I returned two years after the eruption, but I missed out on all the excitement. When the mountain blew its top, a part of me went with it.
From Interstate 5 at Woodland, take the exit for State Highway 503. Drive past Lake Merwin and Yale Lake. Then just after the Swift Reservoir viewpoint, turn left on forest road 83. Follow the signs to Climber's Bivouac, which involves a right turn onto forest road 830.
Please read thoroughly this page from the Mt. St. Helens National Volcanic Monument for information about obtaining permits to climb Mt. St. Helens. A permit is required above 4800' on the mountain. I will not attempt to reproduce all of the red tape requirements on this page. The link above leads to the definitive source of this information.
St. Helens is best climbed during the permit season, with optimal conditions occuring from May through September.
Additional information posted by John: However, don't let winter prevent you from hiking up this stratovolcano. St. Helens is popular from February to early-summer for snowshoe and ski / snowboard summits. A bonus is that you'll get away from the permit quota hassle though you will need a WA or OR Sno-Park permit if you are parking at the Marble Mountain Sno-Park. Check the Books section for Dan Nelson's "Snowshoe Routes : Washington" or Amar Andalkar's Ski Mountaineering and Climbing Site for additional info.
Additional comment posted by ben: Climbing in the winter also means that you can avoid the scree and ash that is the bane of a summer climb - two steps up, one step backsliding.
Please read this page from the Mt. St. Helens National Volcanic Monument for information about camping near the mountain.
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