See if your area has something like 'Dig Safe'. WHere I live, you are supposed to call them before you dig any holes more than x deep. if you don't, and you damage something, you are responsible for all damages and repair costs. It could be a water line, drain line, or protection for some wire or cable. You might be able to find it with a metal detector, but Dig Safe could tell you if it was a utility line of some sort. Depending on where you live, 12" isn't a lot. If that's below the frost line, who knows. Usually, that Dig Safe service is free.