Of 50,000 wells drilled over the past six decades in the Gulf, 23,500 have been permanently abandoned. Another 3,500 are classified by federal regulators as "temporarily abandoned," but some have been left that way since the 1950s, without the full safeguards of permanent abandonment.
Petroleum engineers say that even in properly sealed wells, the cement plugs can fail over the decades and the metal casing that lines the wells can rust.
Leading environmental groups and a U.S. senator on Wednesday called on the government to pay closer attention to more than 27,000 abandoned oil and gas wells in the Gulf of Mexico and take action to keep them from leaking even more crude into water already tainted by the massive BP spill.
The calls for action follow an Associated Press investigation that found federal regulators do not typically inspect plugging of these offshore wells or monitor for leaks afterward.