to me the picture looks like the hole is oval, not round, but that could be an optical illusion from the angle of the picture. Just to toss in another possibility, a small fish pond or fountain pool base. My dad tells me that back in the great depression, people in his neighborhood would raise goldfish and trade them, it was an activity that did not take a lot of money to do. His dad had an outdoor fish pond for the fish in the summer time, I remember seeing the remains of it when I was a child when visiting my grandmother.