It's one of two newly detected 'supereruptions' that rocked North America (and the world) 9 million years ago.

The gargantuan Yellowstone hotspot (also known as the Yellowstone supervolcano) has erupted at least 10 times over the past 16 million years, permanently altering the geography of North America, periodically warping Earth's climate and throwing flakes of airborne ash to every corner of the world.

Now, the discovery of two ancient supereruptions — including the single largest in the hotspot's history — reveals an unexpected trend: The Yellowstone hotspot's activity may finally be waning.

The Yellowstone hotspot is a mysterious blob of hot rock in Earth's mantle, currently sitting under a 1,530 square mile (4,000 square kilometers) stretch of Yellowstone National Park.

Like a big sauce pot sliding over an oven burner, Earth's ever-moving tectonic plates have shifted various parts of what are now Idaho, Nevada, Montana, Oregon and Wyoming over the hotspot during the past 17 million years, leaving a trail of ancient volcanic wreckage behind it.

The most recent Yellowstone supereruption occurred 630,000 years ago and formed much of the modern geography of the park; another occurred below the park 2.1 million years ago.

Researchers have recorded at least four other supereruptions over the past 12 million years, but one 2016 study estimated that at least a dozen supereruptions have occurred since then.

The oldest — called the McMullen Creek supereruption — occurred about 9 million years ago over a 4,600-square-mile (12,000 square km) stretch of what is now southern Idaho, the researchers found.

The second, called the Grey's Landing supereruption, occurred 8.72 million years ago and was absolutely "colossal," the team wrote in the study.

There are now six recorded supereruptions that occurred during the Miocene epoch, between 23 million and 5.3 million years ago.

By comparison, the two supereruptions that occurred since then are separated by 1.5 million years.