When I was very small (back in the 60's), our oil tank suddenly sprunk a leak. My God, the smell was terrible! Dad called the oil company, and Mom and I stayed out of the house all day. Back then, though, all they did was dig out the old dirt beneath the tank and that was it. No major environmental disaster way back then.
Fast forward to our present house. Ten years ago, we moved in and noticed the tank had a slow leak, just enough to need the bottom half of a plastic gallon milk jug with kitty litter in it to absorb the drips. The previous owner had had her oil company put some fiberglass patches and resin on the bottom, but it hadn't seemed to do much good. Just last year, when my Dad and I were replacing some main beam columns, we noticed that the brass oil line fittings on the side of the tank were a bit loose(!). A little bit of gentle tightening and presto! No more leak! No one had ever checked the oil line fittings or valve.
Good thing, too. To take the full size 275gallon tank out, they'd have to empty it, cut it up and move it out in pieces, because when the idiot contractor put the tank in,
he built the basement with no way to remove the tank or put a new full-size one in! There's no hatchway, and the tank won't fit up the basement stairs or out the doorway! Yes, it's the original tank... from 1941.