It has taken me a while, but I have still been researching this image. It's at Lake Lowndes State Park in Mississippi. I spoke with the Ranger and he said he thinks that the prior Ranger had put this in the lake. I don't know if it was put in before the lake was formed, or after. But it is supposedly an old water tank, similar to the photo I have attached. With some plumbing sticking up etc. On its own set of legs. And with rectangular holes cut into the top to let the air out. Make sense? It does to me.