Sometimes events that happened hundreds of thousands or even millions of years ago are scary, not because tons of people were there to see them, but because they are predictive of what might happen in the future.
Take the Yellowstone supervolcano eruptions, which happened 2.1 million years ago, 1.3 million years ago and 640,000 years ago. The largest of these eruptions was the oldest one, which created a volcanic formation known as the Huckleberry Ridge Tuff. It released a whopping 585 cubic miles (2,450 square kilometers) of molten rock and created a caldera measuring some 60 miles (96.6 kilometers) across. That makes it one of the five largest individual volcanic events in history — nearly 6,000 times larger than the Mt. St. Helens eruption of 1980 [source: Lowenstern et al.].
Yellowstone continues to be an active volcanic zone, as its numerous geysers and hot springs attest. What, then, would happen if there were another such eruption? One of the biggest problems would be the massive amounts of ash thrown into the air, which winds would carry across the United States. The Pacific Northwest and Midwest would be particularly hard hit, resulting in a short-term devastation of agriculture and waterways choked with gray sludge. Such an eruption would also eject large amounts of gasses like sulphur dioxide into the atmosphere, causing a decade-long climate cooling. The resulting changes in rainfall patterns and severe frosts could lead to more widespread crop failure [source: Oskin].