" Out there in the millrace, the rush of water was broad and powerful, and as the current pushed past, it did so with an eerie silence. But if you cocked your body at just the right angle, you could detect a faint thrum, a kind of basal tremor. The frequency of that vibration was impossible for the ear to pick up, but it registered unmistakably on the hairs of the forearms, the wall of the chest, and deep in the belly. This was the muffled resonance of a runaway river, the sub-audible bell-tone of water surging with ungovernable force into the throat of the canyon. Just "

― Kevin Fedarko , The Emerald Mile: The Epic Story of the Fastest Ride in History Through the Heart of the Grand Canyon