I was digging up a bush stump so we can pour a concrete pad for a condensing unit. As I dug I uncovered the top of a metal object approximately 3-4ft long and 1.5 feet wide. It's made of some kind of metal (hard to tell with a fair amount of earth still covering it). It seems flat on the top and no apparent pipes so far. I was thinking it might be an old buried fuel oil container, but from what I've researched its fairly small for that. Any ideas? Additional info: its about 6 inches from my foundation and another 6 from a more recent addition that protrude from the house. I should also mention there was an oil storage tank in the basement but that was at least 6-10 ft away and has no obvious connection to this area.