French mariner Paul-Henry Nargeolet, considered one of the leading experts on the Titanic, is feared to be among five people who went missing on a tourist submersible visiting the wreckage. 

Nargelot was named in a Facebook post by fellow voyager, Hamish Harding, a U.K. billionaire and aviator, a day before the mission. 

Paul-Henry Nargeolet,

FILE: Paul-Henry Nargeolet, director of a deep ocean research project dedicated to the Titanic, poses inside the new exhibition dedicated to the sunken ship, at 'Paris Expo', on May 31, 2013, in Paris. (Paul-Henry Nargeolet,)

"The team on the sub has a couple of legendary explorers, some of which have done over 30 dives to the RMS Titanic since the 1980s including PH Nargeolet," Harding wrote Saturday. 

Born in Chamonix, France, Nargeolet is considered a "leading authority" on the Titanic. He is listed as the Director of Underwater Research for E/M Group and RMS Titanic, Inc.

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According to the website, Nargeolet has led several expeditions to the Titanic, completed dozens of dives in the submersible himself, and supervised the recovery of thousands of artifacts, including a 20-ton section of the Titanic’s hull. 

Nargeolet spent more than two decades in the French Navy, rising to the rank of Commander. He retired in 1986 and joined the French Institute for Research and Exploitation of Sea, leading deep submersibles. In this role, he led the first recovery expedition of the Titanic in 1987. 

Paul-Henry Nargeolet

Paul-Henry Nargeolet, director of a deep ocean research project dedicated to the Titanic, poses next to a miniature version of the sunken ship inside a new exhibition, at 'Paris Expo', on May 31, 2013, in Paris.  (JOEL SAGET/AFP via Getty Images)

Per The Irish Examiner, Nargeolet has "spent more time than any other" at the wreckage of the Titanic. Nargeolet ominously told the outlet in 2019: "If you are 11m or 11km down, if something bad happens, the result is the same." 

"When you’re in very deep water, you’re dead before you realize that something is happening, so it’s just not a problem," he said. 

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The submersible, operated by OceanGate Expeditions and carrying five people to document the wreckage of the Titanic, was reported overdue Sunday night about 435 miles south of St. John's, Newfoundland, according to Canada's Joint Rescue Coordination Centre in Halifax, Nova Scotia.

A rescue operation was underway deep in the waters of the Atlantic Ocean Monday in search of the vessel. Lt. Cmdr. Len Hickey said a Canadian Coast Guard vessel and military aircraft were assisting the search effort, which was being led by the U.S. Coast Guard in Boston.

Remains of a coat and boots in the mud on the sea bed near the Titanic's stern

FILE - This 2004 photo provided by the Institute for Exploration, Center for Archaeological Oceanography/University of Rhode Island/NOAA Office of Ocean Exploration, shows the remains of a coat and boots in the mud on the sea bed near the Titanic's stern.  (Institute for Exploration, Center for Archaeological Oceanography/University of Rhode Island/NOAA Office of Ocean Exploration, File)

According to the Coast Guard, the craft submerged Sunday morning, and its support vessel lost contact with it about an hour and 45 minutes later.

David Concannon, an adviser to OceanGate, said the submersible had a 96-hour oxygen supply starting at roughly 6 a.m. Sunday. 

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The expedition was OceanGate's third annual voyage to chronicle the deterioration of Titanic, which struck an iceberg and sank in 1912, killing all but about 700 of the roughly 2,200 passengers and crew. Since the wreckage's discovery in 1985, it has been slowly succumbing to metal-eating bacteria. Some have predicted the ship could vanish in a matter of decades as holes yawn in the hull and sections disintegrate.

The Associated Press contributed to this report.