Turns out, we live in the general vicinity of a petroleum pipeline. So we are advised (and actually, required by state law) to hold our digging, until the exact location of the pipeline has been marked with little yellow flags. This doesn't only go for serious excavations (as in, building a house, if anyone here built basements), but also putting up a fence and the like.
Luckily, it doesn't actually cross our rental property, or it would look as disturbing as in this picture on a flyer we received recently in the mail:
There's little that would make me feel more fuzzy and warm on the inside than have my kids run around and play just a few feet above a dangerous oil pipeline.