In the last 150 years, prospectors and energy companies have drilled as many as 12 million holes across the United States in search of oil and gas. Many of those holes were plugged after they dried up. But hundreds of thousands were simply abandoned and forgotten, often leaving no records of their existence.

Government reports have warned for decades that abandoned wells can provide pathways for oil, gas or brine-laden water to contaminate groundwater supplies or to travel up to the surface. Abandoned wells have polluted the drinking water source for Fort Knox, Ky. , and leaked oil into water wells in Ohio and Michigan. Similar problems have occurred in Texas, New York, Colorado and other states where drilling has occurred.

In 2008, gas from an abandoned well leaked into a septic system in Pennsylvania and exploded when someone tried to light a candle in a bathroom, killing the person, according to a 2009 draft report by the state's Department of Environmental Protection. That report also documented at least two dozen other cases of gas seeping from old wells, including three where the drilling of new wells "communicated" with old wells, leaking gas into water supplies and forcing the evacuation of a home.

In February, methane from an old well made its way into the basement of a house in West Mifflin, Pa., triggering a small explosion. Two families were evacuated and have not yet returned home.

Such incidents rarely receive much attention outside the states and neighborhoods they affect. But as the nation's latest drilling boom continues, abandoned wells have begun attracting more attention, particularly in states where the earth is already pock-marked with holes left by earlier waves of extraction. New wells sometimes disturb layers of rock and dirt near fragile old wells, leading to new cases of contamination.

More Than 1 Million Wells Need Plugging

The most recent effort to count the nation's unplugged wells was a survey published in 2008 by the Interstate Oil and Gas Compact Commission, a multistate agency made up of regulators and industry representatives.

It found that states had located nearly 60,000 wells that needed to be plugged — and estimated that as many as a million more may be out there. In Pennsylvania alone, regulators estimate that 184,000 wells were drilled before records were kept. Many of those wells were plugged with stumps, rocks or nothing at all.

"The fact that there are thousands of these out there that need to be addressed, it's a problem, and it's a problem common to all states" with a history of drilling, said Bradley Field, who heads New York's Division of Mineral Resources.

The task of finding, plugging and monitoring old wells is daunting to cash-strapped state governments. A shallow well in good condition can sometimes be plugged with cement for a few thousand dollars. But costs typically run into the tens of thousands, and a price tag of $100,000 or more isn't unusual.

In the last decade, New York has managed to plug only about 125 of its estimated 40,000 deteriorating wells. It has taken Kentucky more than two decades to plug about 4,000 wells — and it has a waiting list of almost 13,000 more. Even Texas, which has invested heavily in abandoned wells, is years away from plugging all of its open holes. Since 1984, it has plugged more than 30,000 wells. But almost 10,000 are still open, and more are found and added to the list all the time.

Numbers to Grow with Drilling Boom