It's a bathtub/fish pond/ raccoon feeding station.

Keeping with an Ozark stereotype that if we have a bathtub, it may not actually be in the house, we turned this tub into something we thought was more interesting (to avoid taking it to the dump). For a while, some really fancy and expensive fish liked it. Later the local raccoon liked the fish. So, now only the racoon bathes here. Even a welded wire frame didn't stop his nightly raids. So, at present, we are without fish. But we have a nice fat raccoon.