Couple questions... What's the roundish thing on top of it? How thick is the concrete? It could be a capped off well, as suggested, but there's no way to know without digging it out. If it is an old well, at least you'll know it's there and you can cap it of properly to make sure it'll never be a problem. Personally I'd have to dig it out just to see what it was, there's no way I could leave it alone.