When the mountain snows melt, and the springtime runoff swells the banks of this narrow lake situated at the summit of Craig Pass in Wyoming’s Yellowstone National Park, something unusual happens.

Sitting astride the Continental Divide at 8,262 feet (2,518 m) is Isa Lake, a pristine sliver of water spanned by the lower end of the park’s Grand Loop Road. Not only is Isa acknowledged to be the only natural lake in the world that drains to two oceans*, but it also unique in that it does so backwards. (*Part of the Panama Canal, Gatun Lake is a large artificial lake which also flows into both the Atlantic and Pacific Oceans.)

According to the 1905 book, The Origin of Certain Place Names in the United States, the lake was “named for Miss Isabel Jelke, of Cincinnati,” though any reason as to why has been lost to history. Throughout the year, the western end of the lake flows into the Firehole River, which meanders east to the Missouri and Mississippi Rivers and eventually to the Gulf of Mexico. In spring, the eastern end runs into nearby Shoshone Lake, which then drains, via a very convoluted path, into the Lewis, Snake, and Columbia Rivers on the way west to the Pacific.