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A tunnel on the D.C. metro filled with smoke Monday afternoon, killing one woman and sending more than 80 people to hospitals. The Washington Post reported it took about 40 minutes before firefighters could evacuate the trapped passengers from inside six subway cars.

By the time they did, the air had grown thick with smoke:

The preliminary cause of the incident, the National Transportation Safety Board said, was electrical arcing.

Arcing isn't an unusual phenomenon when it comes to industrial sites, transportation safety expert Carl Berkowitz told me. “It’s two pieces of metal, and an electric charge goes through,” he said. “The current will go between where it is being transmitted to where it isn’t being transmitted. If the two pieces are close enough, the electrons will jump. It usually doesn’t cause a problem.”

In fact, you can try this at home (though it’s advisable you don’t): Take two nails, send an electric current through one, and bring the other closer to the first nail. A burst of flame-like flash will appear between the heads as the electrons move from the first nail to the second.

With larger sources of discharge, though, these flashes can create a pressure wave called an "arc blast" powerful enough to injure anyone in the vicinity:

Trains generally see this phenomenon happen without incident—if you've ever seen sparks fly when a train switches rails, that's electrical arcing—but as the NTSB found, the malfunction that occurred Monday came from the line's third rail. The third rail provides the current to power the train through the train's "shoe," or metal contact block that picks up the electricity.

That only solves part of the problem: NTSB officials said the "electrical arcing event" occurred about 1,100 feet in front of the train. What caused enough smoke to stifle the tunnel, therefore, is what Berkowitz said is the main mystery behind the incident.

“That’s what I don’t understand,” Berkowitz says. “The charge happened ahead of the train, and smoke filled the tunnel. So what was at the head of the train?” Some likely guesses so far: water, debris, or enough garbage to create an arc.

In the meantime, it’s too early to tell exactly what happened. Berkowitz said electrical arcing is a solid first step, but according to the NTSB, the investigation could take months to conduct.

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