We’ve heard it several times since the Deepwater Horizon oil rig exploded: Why didn’t the blowout preventer work? It turns out, though, that blowout preventers may not be the failsafe that the oil companies pretend they are. The New York Times reports that “As it turns out, records and interviews show, blind shear rams [a type of blowout preventer] can be surprisingly vulnerable.” A 2000 report found that a jam or leak in a single valve could shut down the entire preventer, and a 2001 study by Transocean, which operated the Deepwater Horizon rig, found a failure rate of 45 percent. BP now says that every oil rig it contracts out comes with two blind shear rams, but Deepwater Horizon only had one—which BP says is because it had to carry its blowout preventer as it moved from well to well. The Minerals Management Service was supposed to require data to show that each rig’s blowout preventers would work, but the man in charge of Deepwater Horizon’s permit admits he approved it without proof.