It used to be that utility poles were everywhere in our commercial and residential neighborhoods. Wires running hither and yon, making quite a mess of things. More and more, everything's underground now: electrical, telephone, water, sewer, gas, and more. That's great as far as the aesthetics go, but that presents a potential problem for anybody who has to dig a hole in the ground. Planting a tree? Building a garage? You run the risk of breaking into one or more of these buried services.
At very least, it makes a mess—who wants to have their nice new trench fill up with the contents of the neighborhood sewer? Break a phone line and your neighbors will be inconvenienced. Hit a buried power line, and that may be the last project you ever undertake.
Call 811 from anywhere in the country, and within a day or two representatives from your local utility companies will come out and mark their buried lines for you. Once you know where these lines are, you know where it's safe to dig.
For more information on when and why to call 811, there's a website to visit as well.