This is a Metate, an ancient grinding stone left over from the Sinagua people that inhabited Central Arizona as far back as Neolithic times. As I was photographing this, a small boy who was visiting the site with his mother asked what this was. She promptly told him it was a basin for water. I chimed in and explained that it was used for grinding corn. It just happened to be filled with water from a recent rain storm. I always wonder what all the tourists think when they visit Indian ruins.