Five years ago Monday, when there was an explosion on the Deepwater Horizon, an oil rig in the Gulf of Mexico, the first news reports said that nearly a dozen people had been killed by the blast.
Before long, however, it was clear that the impact would continue to be felt, and by many more people. The oil spill that began that day and continued into the summer would end up being the worst such accident in U.S. history, spilling millions of gallons of crude into the fragile waterway. How it would be cleaned up remained a mystery, one that is still being answered today.
Another mystery, as TIME’s Bryan Walsh explained in a cover story shortly after the spill began, was what had actually happened on that fateful day:
Investigators are still exploring exactly what went wrong on the Deepwater Horizon, but the catastrophe seems to have been the result of a cascading series of failures–and too little oversight. Rigs are equipped with blowout preventers, 40-ft.-high (12 m) stacks of machinery with multiple hydraulic valves that are designed to seal a well should anything go wrong. Crew members on the Horizon couldn’t activate the blowout preventer, and a deadman’s switch that should have kicked in when control of the rig was lost failed as well. One safety feature the Horizon did not have is an acoustic switch, an additional backup that can activate the blowout preventer remotely. Regulators don’t mandate them in the U.S., though they are effectively required in nations like Brazil and Norway.
When the rig sank, the riser–the pipe that runs from the wellhead to the surface–fell as well, kinking as it did and causing three breaks, from which thousands of barrels of oil are leaking each day. “There were multiple chances to stop this,” says Malcolm Spaulding, a professor of ocean engineering at the University of Rhode Island. “And they all failed.”
Read the full cover story, here in the TIME Vault: The Big Spill
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Write to Lily Rothman at lily.rothman@time.com