" We are three quarters of a mile in the depths of the earth, and the great river shrinks into insignificance, as it dashes its angry waves against the walls and cliffs, that rise to the world above; they are but puny ripples, and we but pigmies, running up and down the sands, or lost among the boulders. We have an unknown distance yet to run; an unknown river yet to explore. What falls there are, we know not; what rocks beset the channel, we know not; what walls rise over the river, we know not. Powell wasn’t overstating their ignorance. At this point, they had no clear idea how far they had come or how much canyon lay ahead of them. They did not know how many turns the river would make, how many rapids there might be, or whether their supplies would sustain them through the time it would take to negotiate these obstacles. And they had no way of knowing that their most serious challenges lay ahead. Just "

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