Isa Lake is believed to be the only natural lake in the world which drains to two different oceans.[3] (For similar cases see List of unusual drainage systems.) 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.[5] 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.