A couple of years ago I was mowing my lawn and the mower barely hit the gas line coming out of the ground to the has meter. It snapped right off at ground level and began spewing gas everywhere. Lucky the mower didn't start a fire. Evacuated the neighborhood for a couple of hours. Turned out that right where the soil and the air meet the gas line had rusted completely through. Three inches above ground no corrosion and three inches below ground no corrosion. Me tapping it with the mower combined with the weight of the gas meter from above and the damned thing snapped right off. Apparently this is fairly common with older beach houses. Got lucky.
Lawnmower will do it, all right. You're really lucky - spark from the mower would have ruined your day.
I will add that DigAlert is absolutely fuck-all useless. But you need to cover your ass liability-wise so you call them. Even if most of the time, they come out 6 hours after the job started (despite having at least a week notice) only to say, wow, did you start drilling already, I wonder if there's anything there.
The worst thing to hit is an electrical line since you'll die. Second worst is a fiber-optic line - that's grab the passport and head to Mexico time because you might as well be dead. Usually they dye top of the trench backfill for the important ones.