Let me add to the scare stories. I work at one of the places that recycles the old, rusty, dusty, uncertified tanks. (It is a scrapyard). We received a rough looking 20 lb tank, with an conglomeration of corroded pipe & nipples on the valve. One of our team recognized it as a waste tank from meth production, that had explosive characteristics. We had to quarantine the area, the bomb squad removed it, and then it was eventually disposed of (very expensively!) as hazardous waste. I tell this short story as a warning about picking up rusty, dusty unknown tanks from a forrest or field.