I guess nobody really knows the exact moment it happened six million years or so ago, but on Feb. 26, 1919, the Grand Canyon was designated a National Park, thus protecting it from development, except for the virtual jam-packed city on its south rim. (Not that I am dissing the south rim; I had one of the best showers of my life at the campground's pay showers there.)
It's not the deepest canyon in the world (both Idaho and Mexico have deeper ones). But for sheer, jaw-dropping vastness, it's hard to beat.
I've been down in the canyon a few times, and not down those easy Bright Angel/Kaibob trails, either. (Let's just say that these days my knees would have issues with the pounding they got on the Thunder River/Deer Creek loop, even though I hiked it not all that long ago.) Once you've been down and back, you start reading about it. I got on a roll with Edward Abbey's essays, as does everyone who's ever been to the Southwest. They're great. And I've read a few interesting histories as well: Stephen L. Pyne's How the Canyon Became Grand: A Short History and FIre on the Rim: A Firefighter's Season at the Grand Canyon.
I can also recommend the late Colin Fletcher's The Man Who Walked Through Time - he walks through the Canyon - with a big caveat: Remember he published it in 1968, when nobody stopped to think it was an atrocious idea to, like, sleep in Indian ruins, thus destroying them, though perhaps they should have, being hippies and all. But then, Abbey liked throwing beer cans out the window, so...