A Yellowstone super-eruption would begin not with a sudden surprise, but with weeks of increasingly alarming warning signs such as migrating earthquake swarms, rapid ground uplift, changing geysers, and rising volcanic gases. In a worst-case scenario, those signals could culminate in a VEI 8 eruption that blasts ash tens of kilometers into the atmosphere, unleashes deadly pyroclastic flows, buries parts of the western United States in ash, and cripples transportation, electricity, water systems, agriculture, and communications across much of North America.