May 18, 2015 marks the 35th anniversary of Earth’s largest terrestrial landslide in historical times—a result of a restless volcano and a uniquely violent eruption. The top of Mount St. Helens plowed into Spirit Lake, throwing water 860 feet above lake level, a great inland tsunami. A ground-hugging hot surge sped across valleys and ridges, killing dozens of people and nearly all other life as it leveled 234 square miles of forest.