In 1999, FEMA identified four hazards in the United States that, were they consummated in all their destructive wonder, would be worthy of the title “catastrophic.” They were: a major earthquake hitting Los Angeles, a major hurricane hitting Miami, a major hurricane hitting New Orleans (check), and a giant earthquake hitting the Central US…

Not far from Memphis and St. Louis, 600 million years ago, the continent tried to rip in half. It failed. But even calling this “the continent” is misleading: North America was probably south of the equator, almost upside down, and had just fractured out of a larger supercontinent called Rodinia. East Antarctica was to the northwest, eastern New England and the Canadian Maritimes were perhaps on a different part of the planet, stitched onto Africa, and a piece that was about to break off of “North America” would eventually end up in the middle of Argentina. This was not our world. This world was still convalescing from the near-death experience of Snowball Earth—having hurtled in and out of planet-wide glaciations, and the first strange whispers of simple animal life had just begun to be etched in ocean rocks.

As the supercontinent Rodinia blasted apart, an ocean not unlike the Atlantic (only a half-billion years earlier) began to grow between the rifting continents. But in “Missouri,” a similar ocean rift stalled. Here the continent tried to divorce but it didn’t take. For hundreds of millions of years this failed rift (called the Reelfoot Rift) has languished as a jagged scar buried deep in the earth. As the ages have passed, it’s been covered far above at the surface by trilobites in shallow seas, later by coal swamps, and later still, by coastal dinosaurs. But it’s never fully healed.

After eons of continental tango, only a few thousand years ago, this rift reawakened. A little over 200 years ago, several faults along this unthinkably ancient gash ruptured, triggering some of the largest earthquakes in American history. Luckily, almost no one lived near Southeast Missouri at the time. But today millions of people do.