From the NYT, the best explanation so far of the BP disaster

It was the last line of defense, the final barrier between the rushing volcanic fury of oil and gas and one of the worst environmental disasters in United States history.

Its very name — the blind shear ram — suggested its blunt purpose. When all else failed, if the crew of the Deepwater Horizon oil rig lost control of a well, if a dreaded blowout came, the blind shear ram’s two tough blades were poised to slice through the drill pipe, seal the well and save the day. Everything else could go wrong, just so long as “the pinchers” went right. All it took was one mighty stroke.

On the night of April 20, minutes after an enormous blowout ripped through the Deepwater Horizon, the rig’s desperate crew pinned all hope on this last line of defense.

But the line did not hold.

For days, technicians and engineers worked furiously to figure out why, according to interviews and hundreds of pages of previously unreleased notes scrawled by industry crisis managers in the disaster’s immediate aftermath.

Engineers sent robotic submersibles 5,000 feet deep to prod the blind shear ram, nestled in the bosom of a five-story blowout preventer standing guard over the Macondo well.

They were driven on, documents and interviews reveal, by indications that the shear ram’s blades had come within a few maddening inches of achieving their purpose. Again and again, they tried to make the blades close completely, knowing it was their best chance to end the nightmare of oil and gas billowing into the Gulf of Mexico.

“If that would’ve worked,” a senior oil industry executive said of the blind shear ram, “that rig wouldn’t have burned up and sunk.”