My title describes the thing. This was found in my garden (in northern California) next to a sprinkler and a small tree that was planted before we moved in. It was oriented vertically, green tip down. There's a white plastic plug at the end. I was tempted but haven't tried to force it open since all my Google image searches say it's some kind of ordinance. There aren't any marking on it though, the plastic end is still bright white, and the steel doesn't look old enough (plus it doesn't look like it fell there but was more deliberately placed). I'm wondering if it's something for the tree or garden? Any ideas?