The phenomenon of the Reversing Falls is caused by the tremendous rise and fall of the tides of the Bay of Fundy, which are the highest in the world. The natural southward-facing opening of the Bay of Fundy receives the on rushing ocean tides directly like a funnel. The tidewater is normal when it enters the bay at its widest point. But the farther up the bay it travels the more it changes. It is, in effect, squeezed by the ever-narrowing sides and the constant shallowing of the bottom forcing the water higher up the shores. The low tide running out of the bay collides with the new, incoming high tide, combining forces to make a higher wave coming in. The combination of wave forces is called resonance.