Small leak in 1 inch buried PVC water line
Start digging. And next time, use 160 or 200 PSI black polyethylene (NSF-PW rated - ie, suitable for potable water, per Fiasco's comment) rather than PVC. (You may not "need" the pressure rating, but it gets you a thicker wall which makes the pipe even more durable .vs. the normal 100 PSI stuff. And digging is something you don't want to have to do over.) 

In any case PE pipe is much more durable than PVC when faced with rocks, movement, etc.