One theory is that the river flows along an underground fault and comes out somewhere under Lake Superior. This is unlikely, because for this to happen, the fault would have to be precisely oriented towards the lake, and would have to be large enough to allow the flow of half the river. Even if such a fault exist, it would have likely been clogged over the years as rocks, sand, logs and other materials fell into the kettle. Besides, there is no evidence of such a fault in the area.
Another theory is millions of years ago a lava tube formed when the rocks first solidified. The problem with this theory is that the rock at Devil’s Kettle waterfalls is rhyolite, and lava tubes never form in rhyolite. Lava tubes form in basalt flowing down the slopes of volcanoes, and the nearest basalt layer to Devil’s Kettle is located much too far underground to be any kind of factor in the mystery. The existence of a large underground cave is also ruled out because underground caves form in limestone rock, and there are no limestone in the area.
The mystery is compounded by the fact no floating debris suddenly appearing at one spot offshore in Lake Superior has ever been reported.