When Mount St. Helens erupted in the spring of 1980, over 200 miles of plants and trees were wiped away by wind, lava, and rock. Nearby buildings were buried, and 57 people were killed. A serene and picturesque landscape had turned terrifyingly barren. But researchers have since found a silver lining: the eruption created a great natural experiment. The federal government designated the mountain a national monument, which allowed plants and animals to return naturally — rather than being placed there by people.