A supervolcano beneath Yellowstone National Park could erupt with catastrophic consequences, beginning with weeks of warning signs. These might include clusters of small earthquakes, ground swelling, changes in geyser behavior, and increased volcanic gases. In the worst-case scenario, this could lead to a VEI 8 eruption — the largest category on the volcanic explosivity index — sending ash tens of kilometers into the atmosphere and triggering deadly pyroclastic flows. Such an event would bury parts of the western U.S. in ash, disrupt transportation, electricity, water systems, agriculture, and communications across North America, and have global effects. Around 631,000 years ago, a similar eruption in what is now north-western Wyoming reshaped the landscape and had worldwide effects. The ash from that eruption spread across North America and affected climate patterns globally, though no humans were present at the time. Today, we know that such an eruption would be a major threat to modern civilization. The first signs of unrest might be detected by scientists monitoring Yellowstone’s seismic activity. In the months before an eruption, a series of small earthquakes might be recorded by seismometers, with patterns suggesting that magma or pressurized fluids are moving beneath the surface. GPS stations might detect ground uplift, and gas sensors could register changes in emissions. While these signs are not always linked to an eruption, they would prompt scientists to increase monitoring and share findings with emergency management. In previous years, similar patterns have occurred without an eruption, but the combination of these signals would raise concerns. Scientists would work closely with agencies like the National Park Service and the Federal Emergency Management Agency (FEMA) to assess risks and prepare for possible scenarios. As the situation escalates, the likelihood of an eruption could rise sharply. Earthquakes might become more frequent and shallower, and geyser activity could become erratic. Gas emissions might increase, and the ground could rise rapidly. At this point, the U.S. Geological Survey (USGS) might raise the Aviation Color Code to orange, indicating heightened volcanic activity. An evacuation zone could be established, affecting hundreds of thousands of people. The global response would depend on whether countries choose to cooperate or act in self-interest. Evacuation efforts could be hampered by limited resources, political disagreements, and the difficulty of moving large populations quickly. If the eruption occurs, the immediate effects would be devastating. Shallow earthquakes would trigger explosions that send superheated steam, ash, and rock into the air. Pyroclastic flows — fast-moving clouds of gas, ash, and rock — could destroy everything in their path, causing widespread loss of life. Ashfall would blanket the region, disrupting infrastructure, agriculture, and communication systems. Over time, the eruption could lead to a global cooling effect as sulfur dioxide in the atmosphere forms aerosols that reflect sunlight. This could disrupt weather patterns, lower crop yields, and trigger economic and social crises. While the eruption would not end humanity, it would reshape the world for generations, forcing societies to adapt to a new reality of limited resources and environmental change.