The frosty volcanic peaks of the Pacific Northwest stand in a remarkably straight line, rising from the crumpled landscape east of Interstate 5. But one volcano is conspicuously out of place. More than 25 miles to the west of the other explosive peaks, in the southwest corner of Washington State, sits Mount St. Helens.

It’s been 40 years since Mount St. Helens famously roared to life, sending ash and gas 15 miles high, flattening 135 square miles of forest, and killing 57 people in the country’s deadliest eruption. Today, the volcano is still one of the most dangerous in the United States, and the most active of the Cascade Range.

Where all this firepower comes from, however, has been