The Grand Canyon of the Yellowstone, up to 900 feet deep, with the Lower Falls dropping 308 feet — nearly twice the height of Niagara Falls. Painter Thomas Moran made the sketches here that helped convince Congress to protect Yellowstone in the first place. Looking at this, it's easy to see why.
© Brocken Inaglory, CC BY-SA 3.0, via Wikimedia Commons