There's a phone number everyone must call before digging in your yard.
That number is 811.
The website:
www.call811.com
What happens when you call 811 is a request goes out to all the utilities and they send locators out who will mark the utilities at your property line. If the mark is near where you are digging, be very, very careful digging around it.
If it's an emergency, they get right on it. Otherwise, it's a 48 hour turn around.
For example, the picture is a typical yard (happens to be in Fairhaven) within a few inches of the public sidewalk. The yellow pipe is gas, the small white pipe is unknown (!), the large white pipe is drainage, and the old rusty pipe is their water main, which the plumbers replaced.
All those pipes were less than 8" below ground level. Scary stuff.
Remember 811 and it is 100% free.