Not being one to believe in wild conspiracy theories, (Ok, so I read them sometimes) here is one that almost makes sense (almost).
The BP Oil spill was sabotage by U.S. Oil companies trying to eliminate BP from bidding on U.S. offshore Oil drilling contracts!
and the less believable part..
The sabotage was performed on the oil rig through Microsoft Windows software that was used to control the valves. The cyber-attack on the rig was executed from onshore.
UPDATE 2: This is getting bad Social-engineering contest reveals secret BP info