We found an old cast iron bathtub in one of our barns when we bought this place. It looks like someone was using it to mix concrete in...go figure. Anyway, we have discussed digging a pit, filling it with rick and sand sort of like a leach field and burying the tub as a "pond" for our ducks. Maybe they did something like that and it just filled in over the years.
I was a little scared when we were pulling black pieces of plastic like garbage bag, but I think you are right about that they tried to line the bottom with plastic.