California is facing a potentially devastating natural disaster that scientists say is not a question of if, but when. Researchers have warned that a massive series of atmospheric rivers could unleash a catastrophic “megaflood” across the state.

Known as ARkStorm 2.0, the scenario envisions powerful storms repeatedly striking California over several weeks. Such an event could overwhelm rivers, reservoirs, levees and drainage systems, producing widespread flooding across the Central Valley and other heavily populated areas.

California has experienced similar disasters before. During the historic Great Flood of 1861–62, relentless storms battered the state for roughly 45 days, turning large portions of the Central Valley into an inland sea. Scientists have found geological evidence that enormous floods have occurred repeatedly throughout California’s history.

The modern consequences could be staggering. USGS modeling has estimated that a severe ARkStorm could force more than a million people to evacuate and cause hundreds of billions of dollars in damage. Updated scenarios have placed potential economic losses at more than $1 trillion.

Scientists emphasize that this is a plausible disaster scenario, not a prediction that a megaflood is about to happen. There is currently no such storm developing. But the warning is clear: California has experienced extreme megafloods before, and another one will eventually occur.

The question may not be whether California will face another Great Flood—but whether the state will be prepared when it comes.