Some 15 cities along a 200-mile stretch of the Pacific Northwest were coated in a mysterious, chalky substance on Feb. 6 after "milky rain" fell from the sky.

For months, the white substance has been considered a "good old-fashioned weather mystery," per CNN. Now, it's a mystery no longer. Though some have suggested a Nevada dust storm or ash from wild fires or far off volcanoes was responsible, Washington State University scientists who analyzed samples of the rain along with meteorological data say the guilty party is a shallow saline lake 480 miles away in Oregon.

Soil from Summer Lake was thrown into the air by a bad dust storm and carried north by strong winds. When the clouds opened up, the particles fell to the ground with rain over various parts of Oregon, Washington, and Idaho, the scientists explain in a press release.

"At first we suspected it was related to wind erosion of landscapes that had previously burned, but the wind trajectory analyses didn't add up," a researcher says.

Wind patterns showed the substance couldn't have come from suspected volcanic eruptions in Russia and Mexico, either. "Instead, the air flow was locally from the south," a meteorologist says.

The scientists traced the milky matter to Summer Lake—known to dry up during droughts, Christian Science Monitor reports—where 60mph winds were recorded the night before the rain poured.

"That would have been powerful enough to lift a good-size dust plume," the meteorologist says. The Tri-City Herald reports samples included sodium, silicon, and trace amounts of aluminum, potassium, magnesium, calcium, iron, and manganese.

"The chemistry is consistent with a saline source from a dry lake bed," a hydrochemist adds. (Find out how raindrops get their smell.)

This article originally appeared on Newser: Northwest's 'Milky Rain' Mystery Is Solved

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