With down-hole temperatures of 440 degrees F. and pressures of 27, 000 pounds per square inch, such depths proved too daunting and expensive for Exxon-Mobil (XOM), which sold its Blackbeard well, at the mouth of the Mississippi, to McMoRan in 2008.

FORBES: Bullish On U.S. Oil And Gas? Consider These 'Stocks Under Rocks'

Plugging the hole could be "astronomically expensive, " says Edmund F.

FORBES: Magazine Article