"A "black box" can reveal why an airplane crashed or how fast a car was going in the instant before an accident. Yet there are no records of a critical safety test supposedly performed during the fateful hours before the Deepwater Horizon oil rig exploded in the Gulf of Mexico.

"They went down with the rig.

"While some data were being transmitted to shore for safekeeping right up until the April 20 blast, officials from Transocean, the rig owner, told Congress that the last seven hours of its data are missing and that all written logs were lost in the explosion.

"The gap poses a mystery for investigators: What decisions were made and what warnings might have been ignored? Earlier tests, which suggested that explosive gas was leaking from the mile-deep well, were preserved."

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