A version of this story was co-published with the Pittsburgh Post-Gazette.

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.

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.

Some regulators fear that the number of abandoned wells will grow when the current drilling boom runs its course. Last year, oil and gas operators drilled almost 45,000 new wells across the United States, and that number is expected to hold steady or increase as the nation tries to wean itself from foreign oil. If even a small fraction of those wells is eventually abandoned, states will be left with the bill, just as they were when the last boom ended in the mid-1980s.

To prevent that from happening, states require energy companies to post bonds before they begin building their wells. But the bonds are often so low that it can be more economical for a company to forfeit its bond rather than plug its wells. In Pennsylvania, for instance, an energy company can cover hundreds of wells with a single $25,000 bond.

John Hanger, who until January headed Pennsylvania's Department of Environmental Protection, called the bonds "scandalously low."

"There are some choices you shouldn't put in front of even good companies," Hanger said. "I'd like to think the companies would do the right thing, but we know that just isn't always the case."

The Birthplace of an Industry

One of Pennsylvania's worst cases of gas migration occurred in the Borough of Versailles, a small, working-class community just outside Pittsburgh. From 1919 through 1921, more than 175 gas wells were drilled in the town. Residents put wells in their backyards to heat their homes, packing them into the 25-by-100-foot lots.

The boom dried up when most of the wells proved unproductive. But in the 1960s, pockets of gas began leaking into homes. Some houses were condemned and demolished, and Versailles eventually became a case study for federal scientists trying to locate old wells.

Researchers studied old maps and walked the grounds with magnetometers, which detect the magnetic field from metal casings in the wells. If casings were never installed or had been removed, they could test the soil for hydrocarbons that might be leaking to the surface.

Some of the old wells were plugged. But more often vents were installed to direct gas away from the homes. Today, dozens of pipes pop out of the ground in yards, behind garages and through houses, slowly leaking methane and hydrogen sulfide so the explosive gases don't accumulate. In 2009 Versailles received a $368,600 federal grant to maintain its aging vents. About 50 methane alarms have also been installed in the town.

The vents and alarms are just part of life in Versailles. The mayor, James Fleckenstein, recently bought a house with two vents on the property and an alarm in the kitchen.

"We've been living with this problem forever," Fleckenstein said. "People would have a vent in their yard burning 24 hours a day all year long, a one-inch pipe sticking out of the ground. People would put a coffee can and light it and it would just burn all the time."

There's no longer enough pressure in the gas formation to make the vents flammable, Fleckenstein said, and the town hasn't had any problems with migrating gas for a couple of years. But that could change at any time, said Fred Baldassare, who for years oversaw gas migration cases for the Department of Environmental Protection and now runs a consulting business. Old wells can deteriorate or become clogged, he said, and conditions underground can change.

In February, gas from an abandoned well caused a small explosion just across the river from Versailles, in West Mifflin, Pa. The gas company evacuated the house where the explosion occurred, as well as the house next door, where Nick Kellington lived with his wife and four children.

"I said 'How long are we packing for?' and he said 'I don't know,'" Kellington said. "Somebody tells you that, what do you do?"

Kellington said the DEP, which declined to comment about the case, used old maps to identify a nearby well that may be the source of the gas. The Kellingtons are renting a townhouse while they wait for a state-hired contractor to fix the problem. Baldassare has been hired as a subcontractor.

Every time the Kellingtons visit their former home, a gas company employee must first test the temporary vent that sticks out of their basement window. Even then, Kellington has to leave his cell phone outside, lest a spark ignite a pocket of gas.

Finding and plugging an old well can be risky, because nearby wells may be disturbed and begin releasing gas. So, the Kellingtons' home is being fitted with a system that pumps air under the house, creating a high-pressure zone that would prevent the gas from leaking indoors.

Kellington said that even if the system is installed successfully, he may try to sell the house and move.

"My wife just doesn't feel safe," he said.

A 150-Year-Old Legacy

Edwin Drake drilled the nation's first commercial oil well near Titusville, Pa., in 1859, and for decades people across the country drilled wells as they pleased. Some states didn't develop modern regulations until the second half of the 20th century.

In the early days, the industry was dominated by "get-rich-quick wildcatters," said Dale Henry, a veteran of the oil and gas industry who ran unsuccessfully to sit on the Texas Railroad Commission, which regulates drilling in the state. Those who didn't get rich often ran out of money before they could plug or seal their wells, Henry said.

"They simply fold their tent, head into the darkness and you never hear or see them again," he said.

Nobody knows how much damage abandoned wells have caused over the years. Most states don't systematically track cases of contamination that result from abandoned wells, said Mike Nickolaus, special projects director for the Ground Water Protection Council, an association of state groundwater agencies.

"It might be a problem, it might not," Nickolaus said. "That's the problem you have. You can't just count up the numbers and say that represents a big problem, a small problem or no problem at all."

Despite the lack of comprehensive data, state and federal reports have chronicled scores of contamination cases over the last couple of decades.

In 1989, the Government Accounting Office found nine cases where abandoned wells had contaminated groundwater, including one Kentucky case that rendered the water for more than 80 households undrinkable. The GAO said its findings were incomplete and warned that state agencies didn't have the funds to track and plug the growing number of abandoned wells.

Ohio's Department of Natural Resources found 41 cases where abandoned wells contaminated private water supplies from 1983 through 2007. Those incidents represented about 20 percent of Ohio's oil and gas contamination cases during that period.

The EPA's 2004 study of hydraulic fracturing, which is best known for determining that fracking didn't threaten drinking water, examined groundwater contamination in the San Juan Basin in Colorado. The report said state authorities suspected that gas migration into water wells and buildings was due, at least in part, to the presence of abandoned wells.

Some regulators are concerned that fracking, which is used in most new wells, increases the possibility that old wells will be damaged or disturbed. The process injects water, sand and chemicals into wells at high pressure to release oil or gas. But by disrupting the earth it can also push gas and other contaminants into openings created by old wells.

That's what investigators think happened on a rainy day in 2006, when gas and water began spewing out of the ground on Emile Alexander's farm in Washington County, Pa. Investigators determined that the fracking of a new well caused methane to leak through an abandoned well and into the ground and the aquifer. Some water wells near Alexander's farm have been fitted with vents to allow stray gas to escape. Methane gas isn't toxic to ingest, but it can explode.

Although Alexander receives royalty payments from two wells on his land, he wishes he had never signed the lease. "I didn't sign it to kill me," he said.

In an internal briefing last year, EPA scientists raised concern that fracking near Pennsylvania's many abandoned wells could threaten groundwater, saying the old wells "may present a risk unique to the hydrofrac process."

This year, as part of its first comprehensive study of hydraulic fracturing, the EPA plans to look at whether abandoned wells might become conduits for fracking fluids.

Lack of Money Thwarts a Fix

More than two decades ago, federal scientists determined that chlorides, or salts, were leaking through abandoned wells and into an aquifer that supplies drinking water to nearly 40,000 people in Fort Knox, Ky.

Mike Unthank, a scientist with the United States Geological Survey, said the problem could be solved if a few dozen abandoned wells were plugged. But the bids that Kentucky's Division of Oil and Gas state received were so high that the Fort Knox project alone would deplete its $1.3 million plugging fund.

Marvin Combs, the division's assistant director, said the fund is used to plug 250 to 300 wells a year. At that rate, it will take more than 40 years to work through the nearly 13,000 wells the state has identified so far.

While the Fort Knox wells wait their turn, local water officials are using a pumping system to push fresh water through the aquifer, diluting the chlorides enough to keep them below the federal safe drinking water limit. But the system must be constantly monitored, and just a few missteps or a change in conditions could make the water undrinkable.

Ideally, the bond money that a company must post before drilling would cover the cost of plugging any wells it abandons. But the bonds are often too low. A recent Government Accountability Office review found that the minimum bonding levels for drilling on federal land were set half a century ago and that Interior Department officials haven't adequately reviewed them or raised them when necessary.

"It begs the question, does it truly provide the incentive to operators to do the reclamation? Because that's the whole point of what the bond is," said Anu Mittal, the GAO report's lead author. "Given how many wells are being drilled right now -- in the last decade it has doubled -- it does raise some concerns that if the operators don't reclaim the land like they're supposed to it's creating a huge liability for the federal government."

Some states, including Wyoming, have strengthened their bonding requirements. But Wyoming's oil and gas supervisor, Tom Doll, said that though his agency now requires some of the highest bonds in the nation, it couldn't afford to plug all the wells that would likely be abandoned if the energy market crashed again, as it did when oil prices fell 50 percent in 1986. He said lower natural gas prices have already driven energy companies to close some coal bed methane wells in Wyoming.

John Hanger, the former head of Pennsylvania's Department of Environmental Protection, said the way states handle today's drilling rush will determine how this boom eventually plays out, especially in the Marcellus Shale, which lies beneath Pennsylvania, New York and parts of six other states.

"I personally think one of the very good things that's coming out of the Marcellus is a whole lot more attention to the problems that were created in the past and the opportunity to do it better this time," he said. "If we don't raise the bonding amounts, we're repeating a mistake."

Nicholas Kusnetz is an investigative reporter for ProPublica, an independent, non-profit newsroom that produces journalism in the public interest.

Follow on Twitter: @nkus