How much water does Lake Supreior contain?

3 quadrillion gallons.

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Want to Escape Global Warming? These Cities Promise Cool Relief. (The New York Times, 4/15/2019)

Or, as the headline reads in the print edition:  As the Earth Warms, Duluth is Looking Pretty Cool.  Duluth, which sits at the western end of Lake Superior, the greatest of the Great Lakes by volume, also has fresh water. A lot of it. Superior is so voluminous that, if poured out, it would submerge North and South America under a foot of water.

Related reading:
Just how big is Lake Superior?   (Lake Superior Magazine)