The State Mine Inspector's office has inventoried over 10,000 abandoned mines in Arizona since 1992 but estimates there may be as many as 100,000 across the state (http://www.asmi.state.az.us/faq.asp).
About 13% of the known mines are considered a significant public safety hazard. Extrapolating, that could mean as many as 13,000 dangerous shafts or pits, most of them unmapped or unknown. Like the one near Chloride where two young girls on ATVs fell in over the weekend, killing one of them.
[right - courtesy State Mine Inspector]
Arizona is lucky if it fences or seals off a dozen abandoned mines a year. The abandoned mine program is small and costs are high to block off the most dangerous locations.
State Mine Inspector Joe Hart, a former state senator, armtwisted the Arizona Legislature into adding $50,000 to his agency's budget this year for abandoned mines. Phelps-Dodge, now Freeport McMoran, contributed a matching $50,000.