Isa Lake is believed to be the only natural lake in the world which drains to two different oceans backwards. (The man-made Gatun Lake in Panama also drains backwards into the Atlantic and Pacific Oceans via the Panama Canal.) The east side of the lake drains by way of the Lewis River to the Pacific Ocean and the west side of the lake drains by way of the Firehole River to the Atlantic Ocean. This is the opposite of what one would expect since the Atlantic Ocean is east of the lake and the Pacific Ocean is to the west.
The lake is easy to visit as it is adjacent to the road that now connects the Old Faithful and West Thumb geysers basins, on what is known as the "lower loop" of the figure-eight roadway which traverses through Yellowstone.