DOWN PERISCOPE: The San Juan’s Final Hours.

In recent days, Argentine authorities have gathered crucial evidence to piece together a theory of what happened after the San Juan left Ushuaia on Nov. 8, bound for its home base of Mar del Plata about 260 miles south of Buenos Aires. They believe the sub took on water that caused a short circuit and a subsequent fire in a key battery compartment. The crew got the fire under control, but hours later there was a loud noise consistent with an explosion near the sub’s last reported location.

Top Argentine officials now believe a blast instantly killed the sailors and sent the vessel to the seafloor. While no one knows for sure what caused the explosion, the batteries are the likeliest culprit, Argentine naval officials and outside experts say.

“It looks like a very possible cause,” said Mike Fabey, a naval expert at Jane’s by IHS Markit, a defense consulting firm. “Battery issues of all types with submersibles are something people have been trying to deal with since submarines first went into the water. Water plus electricity equals fire.”
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A diesel-electric sub’s batteries only hold a charge for about a week while submerged, so it must surface and “snorkel,” running its engines to recharge the batteries and ventilate stale air.

In this case, the sub surfaced amid rough seas, with 23-foot-high waves that may have caused the sub to take on too much water through its snorkel, Argentine Navy spokesman Capt. Enrique Balbi said. The snorkel is equipped with a flap to keep water out, but water can sometimes get in anyway, navy officials said.

Sloppy maintenance or sloppy seamanship would seem the likeliest culprits.