A Mile Deep

The two second explanation of the Grand canyon is...."The river cut down through it and the sides fell in."  
They fell in steeply too.  The bottom is a vertical mile away from the level of the surrounding plateau. 
In the picture you can see people on the edge.  They're just behind my head.  While there are plenty of places that have a safety wall along the edge there are far more places that have been left the way God made them.