Two decades after blowing its top, Mount St. Helens is stirring again. Here is how scientists keep an eye on an active volcano

AS PLUMES OF STEAM SHOT OUT OF ITS horseshoe-shaped crater last week, Mount St. Helens seemed on the verge of a spectacular eruption. And this time the whole world was watching--hundreds through binoculars at safe vantage points, millions more through hourly reports on cable TV. But no one was watching more closely than the scientists monitoring the instruments scattered across the mountain's ash-coated flanks and half-mile-wide lava dome. This early warning network was installed after the 1980 eruption that blew off the top 1,300 ft. of the mountain, destroying tens of thousands of acres of forest and killing 57 people. Although...

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