It started out as a normal day for the Jean Nicoli, a passenger ferry in the south of France. The boat had departed from Porto Vecchio, and pulled smoothly into the dock in Marseille. Guests had disembarked, and crew members were preparing for the next trip. But then, out of nowhere, they felt an unusual sensation:

“I was in the cabin, and there was [a] noise: ‘boom,’” one maintenance worker told Agence France-Presse. “We felt something moving up and down,” said another.

After recent terror attacks, France is on high alert for unexpected violence. But local police also suspected the blast could be a vestige of an older conflict—World War II. The sea around the port is filled with munitions, left there after the 1944 Battle of Marseille.

So after the crew was evacuated and the ferry stalled, they sent divers down to investigate the seabed. But their findings “did not seem to support this hypothesis,” one source told AFP.

As there were no immediate apparent threats, the Jean Nicoli is back in business, sent on its way to Sardinia. But the blast, for now, remains a mystery.

“There was something, for sure, but what?” said the source.

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