Electric power lines, natural gas pipelines, communications lines, and other utility services could be within a few feet of the surface. For homeowners, excavating in your yard poses some risk. This could cut off service not just to your home, but to the entire neighborhood. 

Digging into an underground electric line can cause power outages and personal injury from shock or electrocution.

A damaged gas pipeline or service to a house can create an explosion hazard that potentially endangers both persons and property. These underground facilities are not always located out in the street. So whether you are grading, installing sprinklers, or planting a tree, PostsHoles4you insists you to make the call.

For free locating service, call Ontario One at least 4 days before you plan to begin digging. 

Ontario One Call will take information about your planned digging and communicate it to Enbridge Gas Distribution and other to members who have buried pipes or facilities in your area. Member utilities will then contact you and, if necessary, arrange to send a locator to identify and mark the location of their buried facilities.
Call anytime — 7 days a week and 24 hours a day — at least four working days in advance. 

The number is:1 800-400-2255.
Or go to there web site for more info www.on1call.com/