The Pacific Northwest was home to one of the Earth's largest known volcanic eruptions, a millennia-long spewing of sulphuric gas that blocked out the Sun and cooled the planet 16.5 million years ago, according to a study.
Only two other eruptions - the basalt floods of the Siberian Traps and the Deccan Traps - were larger, and they led to two of the Earth's great extinctions, researchers said.
"This would have been devastating regionally because of the acid-rain effect from the eruptions," said John Wolff, a professor at the Washington State University in the US.
"It did have a global effect on temperatures, but not drastic enough to start killing things, or it did not kill enough of them to affect the fossil record," Wolff added.
Starting 16.5 million years ago, vents in southeast Washington and northeast Oregon put out a series of flows that reached nearly to Canada and all the way to the Pacific Ocean, researchers said.
The flows created the Wapshilla Ridge Member of the Grande Ronde Basalt, a kilometre-thick block familiar to travellers in the Columbia Gorge and most of Eastern Washington.
It is "the largest mapped flood basalt unit on Earth," they said.
The study, published in the journal Geology, estimates that, over tens of thousands of years, the floods put out between 242 and 305 billion tonnes of sulphur dioxide.
That is more than 4,000 times the output of the 1815 Mount Tambora eruption in present-day Indonesia.
That eruption blanketed the Earth in an aerosol veil, creating the "Year Without A Summer" and food shortages across the northern hemisphere, researchers said.
The volume of gas emitted from the Wapshilla Ridge lavas, is equivalent to a Tambora eruption every day for 11 to 16 years, they said.
Most of the lava's gases were released during the eruptions, but some of the gas remained trapped in crystals near the volcanic vents.
Researchers put the eruption into one of three classes of cataclysms, the other two being a caldera eruption like the Yellowstone volcano and the impact of an asteroid.
A similar eruption today "would devastate modern society globally," said Wolff.