Previously, we talked about California’s 1861 super flood that turned its Central Valley into an inland sea.

That flood was caused by an ARkStorm. The A R in Ark stands for Atmospheric River.

Atmospheric rivers are layers of water vapor that form in the tropics and circle the globe.

They’re very large—up to 2 miles high, 500 miles wide and 5,000 miles long. Each can carry 15 times the flow of the Mississippi River.

Atmospheric rivers are always forming, and always flowing—until they hit something like a mountain range, that forces them up into the colder atmosphere, where they condense into rain.

In this way, they provide 90 percent of the rain in the mid-latitudes, and up to 50 percent of California’s.

But they can also bring extreme floods.

Sedimentary records show this has happened in California 10 times in the last 2,000 years, suggesting that California is due for another one.

Since the 1861 flood, the state’s population has increased more than 100-fold. Millions of people now live in areas vulnerable to droughts, fires and floods.

Scientists predict an ARkStorm could flood a quarter of California homes, cause one and a half million people to evacuate, and leave almost a trillion dollars in damages.

A California super flood is as likely as a super quake but could be three times more devastating.