Must of been and old steel tank. Why make them plastic now. The last one I had installed was concrete. I have had two of them on lots I have bought so far, but weren't used for years so they had just dried up and all I had was a hole to fill without any mess.
Two things I have found on lots that have or had old houses are old steel septic tanks and hand dug wells. Two things to watch for or you can end up in them.
That always seems to happen to me week after week run over something with no problems than the one week comes where the sprinkler head didn't go down or the irrigation box lid got loose or the water meter cap didn't get replaced properly and smash!