An organization in the state of Michigan and, if I remember correctly, several other states in the Midwest US, whose purpose is to protect underground facilities. The Miss Dig system is set up to allow for all utility companies in an area to be alerted with a single phone call.
Here in Michigan, (some details vary from state to state) a contractor or homeowner must contact Miss Dig 72 hours prior to digging in the ground. When a call is received, a ticket is generated for that job, containing information on the type of work being done, who is doing it, and where (to varying degrees of specificity). Miss Dig keeps records of the areas covered by each utility company, and forwards the ticket to those in the area in which the work is being done. The individual utilities (telephone, electric, water, etc.) then check their own records to determine more specifically what facilities they have in the area.
The utility companies are then responsible for marking the location of any facilities that may be in conflict with the work being done. For normal tickets, this must be done within 3 business days. For emergency tickets, the deadline is two hours. Some utilities do their own locating, while some contract others to do this for them. I work for one such company that is contracted to locate for certain telephone, electric and natural gas companies in this area. The utilities are marked using paint and flags.
The law requires that no power equipment be used within 18 inches (24 in some states) of the marks. Workers are required to hand-expose (or "pothole", as it's called) the facilities when working within this buffer zone.