More than 27,000 abandoned oil and gas wells remain in the Gulf of Mexico and no one's checking to see if they are leaking, reports an investigation by the Associated Press.

The AP, calling the Gulf an "environmental minefield," says the oldest of these wells were abandoned in the late 1940s, raising questions about whether their seals remain intact. It says 3,500 wells are listed as "temporarily abandoned," without seals, with 1,000 of them remaining that way for more than a decade.

The Macondo well beneath BP's Deepwater Horizon rig was being sealed with cement for temporary abandonment when it blew April 20, according to the AP, which says government data indicate BP alone has abandoned about 600 wells in the Gulf. The story adds:

There's ample reason for worry about all permanently and temporarily abandoned wells – history shows that at least on land, they often leak. Wells are sealed underwater much as they are on land. And wells on land and in water face similar risk of failure. Plus, records reviewed by the AP show that some offshore wells have failed.

Experts say such wells can repressurize, much like a dormant volcano can awaken. And years of exposure to sea water and underground pressure can cause cementing and piping to corrode and weaken....

Oil company representatives insist that the seal on a correctly plugged offshore well will last virtually forever....

Officials at the U.S. Interior Department, which oversees the agency that regulates federal leases in the Gulf and elsewhere, did not answer repeated questions regarding why there are no inspections of abandoned wells.

State officials estimate that tens of thousands are badly sealed, either because they predate strict regulation or because the operating companies violated rules. Texas alone has plugged more than 21,000 abandoned wells to control pollution, according to the state comptroller's office.

Offshore, but in state waters, California has resealed scores of its abandoned wells since the 1980s.

In deeper federal waters, though – despite the similarities in how such wells are constructed and how sealing procedures can fail – the official policy is out-of-sight, out-of-mind.

The U.S. Minerals Management Service – the regulatory agency recently renamed the Bureau of Ocean Energy Management, Regulation and Enforcement – relies on rules that have few real teeth....

Unlike California regulators, MMS doesn't typically inspect the job, instead relying on the paperwork....

Companies permanently abandon wells when they are no longer useful. Afterward, no one looks methodically for leaks, which can't easily be detected from the surface anyway. And no one in government or industry goes underwater to inspect, either....

The AP documented an extensive history of warnings about environmental dangers related to abandoned wells:

_ The General Accountability Office, which investigates for Congress, warned as early as 1994 that leaks from offshore abandoned wells could cause an "environmental disaster," killing fish, shellfish, mammals and plants. In a lengthy report, GAO pressed for inspections of abandonment jobs, but nothing came of the recommendation.

_ A 2006 Environmental Protection Agency report took notice of the overall issue regarding wells on land: "Historically, well abandonment and plugging have generally not been properly planned, designed and executed." State officials say many leaks come from wells abandoned in recent decades, when rules supposedly dictated plugging procedures. And repairs are so routine that terms have been coined to describe the work: "replugging" or the "re-abandonment."

_ A GAO report in 1989 provided a foreboding prognosis about the health of the country's inland oil and gas wells. The watchdog agency quoted EPA data estimating that up to 17 percent of the nation's wells on land had been improperly plugged. If that percentage applies to offshore wells, there could be 4,600 badly plugged wells in the Gulf of Mexico alone.

_ According to a 2001 study commissioned by MMS, agency officials were "concerned that some abandoned oil wells in the Gulf may be leaking crude oil." But nothing came of that warning either.