A riddle haunts the Titan disaster. It’s the presence on the doomed craft of Paul-Henri Nargeolet, 77. The Frenchman was one of the world’s great submariners. So why was he, of all people, diving repeatedly to the Titanic on a submersible that many experts saw as a catastrophe waiting to happen?

“It’s a source of great puzzlement,” said Victor L. Vescovo, a sea explorer who hired Mr. Nargeolet to oversee a series of unusually deep dives.

Weeks before the loss of the Titan and its crew in June, Alfred S. McLaren, president emeritus of the Explorers Club, was hosting a dinner party at the Harvard Club of New York City for Mr. Nargeolet and eight other guests when he learned of his friend’s repeated dives aboard the experimental craft.

“I was speechless,” Dr. McLaren, a retired Navy submariner, recalled. “I wanted to say, ‘What the hell are you doing?’”

Since the June 18 disaster, no single answer to that question has emerged. One theory holds that Mr. Nargeolet believed that his undersea skills would let him manage Titan’s obvious design flaws. Another sees his love for the storied 1912 shipwreck — he was known as “Mr. Titanic” — as blinding him to the danger. But Mr. Nargeolet may have had other motivations as well.

For decades, the submariner was locked in a feud with Robert D. Ballard, an American oceanographer often credited with the wreck’s 1985 discovery. The two men represented different sides of a broader dispute involving governments in opposition across the Atlantic Ocean as well as competing philosophies on shipwrecks, a fray that Washington rejoined in recent days. Both sought the moral high ground. Their conflict centered on whether the ship’s artifacts should be retrieved.

And it never left Mr. Nargeolet’s mind — even into the hours before his final dive.

A Titanic Star Is Born

More than two miles down, in pitch darkness at the bottom of the icy North Atlantic, Titanic lies split in two. The bow rests a half mile from the stern. Nearby are boilers, engines, hatch covers, hull slabs, deckhouse ruins and other large parts that broke free.

The surrounding debris fields hold small, rugged items — lumps of coal, shoes, champagne bottles, jewelry. Bones and bodies disappeared long ago. Overall, the ship’s parts and human paraphernalia extend over roughly three square miles.

Divers cannot see the spectacle at a glance. The enormous scale of the ship’s disintegration starts to hit home only after repeated visits. For Mr. Nargeolet, it was a giant puzzle. He wanted to fit the pieces together and worried that the sea would take them before he could figure it out.

Jean Jarry, a French colleague of Mr. Nargeolet’s who helped find Titanic, said one of the submariner’s dreams was to produce “a complete inventory of the wreck before it was entirely destroyed by marine life.”

Mr. Nargeolet heard the sea’s call as a young man and in 1964, at age 18, joined the French Navy. He dove, cleared mines, served as a submarine pilot and explored shipwrecks.

In 1986, he went to work for the French maritime agency that had helped find Titanic the year before. Starting in 1987, as a submersible pilot on contract to an American firm, wielding robotic arms, Mr. Nargeolet helped retrieve more than 5,500 artifacts.

The finds included chandeliers, a bronze cherub from the ship’s grand staircase, and 65 perfume vials belonging to a first-class passenger. Also raised was a leather bag full of sheet music and love letters. The big haul was a section of Titanic’s hull. It weighed 17 tons and displayed a row of portholes.

In all, Mr. Nargeolet made 38 dives to the shipwreck. He eventually went to work for RMS Titanic Inc. The company, based near Atlanta, had obtained from a federal court in Norfolk, Va., the exclusive salvage rights to the ship’s artifacts. Mr. Nargeolet became the company’s director of underwater research.

He saw the shipwreck as a kind of archaeological site whose treasures could become the showpieces of museums and exhibitions. He cast the displays as important not only for public education but also for commemorating the more than 1,500 people who lost their lives.

Mr. Nargeolet would sometimes tour with the artifacts. In 2017, he traveled to Peoria, Ill., to speak at a traveling exhibition. While there, he gave Bryce Orwig, an 11-year-old boy undergoing cancer treatments, an individual tour. “He made it special,” Bryce recalled.

When not honoring the artifacts, Mr. Nargeolet would visit the sunken liner whenever he could, even if the expeditions had no interest in picking up remains. He simply wanted to see Titanic up close.

In 2019, he advised Mr. Vescovo when the sea explorer dove to the ship’s resting place. The assembled team of maritime experts judged that the liner’s remains were rapidly falling apart, done in by rust, corrosive salts, microbes and deep-sea creatures.

In the years that followed, that worry stuck with the submariner.

“I’m currently on the site of the Titanic,” Mr. Nargeolet said in an email to Mr. Jarry on Saturday, June 17 — the day before he and four others perished on Titan’s last dive. “In my opinion, the future of the Titanic, like all wrecks, is to disappear.”

If the ocean liner’s memory is to be preserved, Mr. Nargeolet added, “now is the moment we must do it.”

A Rift Beneath the Sea

Dr. Ballard, an oceanographer, is often credited with discovering the Titanic. He has made this claim himself in the subtitle of his memoir and when he told a federal judge in Virginia in 2017, “I’m the person who discovered the Titanic.” He has also said that Jean-Louis Michel, leader of the French team in the 1985 expedition, “never gets enough credit” as the shipwreck’s co-discoverer.

Dr. Ballard was a senior scientist at the Woods Hole Oceanographic Institution on Cape Cod when the Titanic was found. He started out on the same side as Mr. Nargeolet. In October 1985, he told Congress that he supported “recovering those delicate items lying outside the hull.”

By definition, such objects would be shoes, leather bags and other small items.

Then, as the French and Mr. Nargeolet recovered such artifacts, as well as parts from inside the ship, he reversed himself. Dr. Ballard became the public voice of preservation, arguing that the Titanic, like the remains of U.S.S. Arizona at Pearl Harbor, should be honored as a great tragedy and left untouched.

He also cast the ship’s salvors as grave robbers. His voice carried far. Starting in 1987, he published a half-dozen books with “Titanic” in the title and hosted a similar number of television shows.

The French, including Mr. Nargeolet, deeply resented being called grave robbers and came to see the American expert as stealing credit for the ship’s discovery.

“He felt a very strong sense of betrayal,” Dr. McLaren said of Mr. Nargeolet. “It was something he talked about repeatedly. It never left his mind.”

Mr. Nargeolet and Dr. Ballard began to battle openly after the American in late 2004 condemned the salvors in a National Geographic article. “Grave site or gold mine?” a photo caption asked. He accused the divers of damaging the famous liner, calling them “bulls in a China shop.”

Mr. Nargeolet fired back. In an open letter, he charged Dr. Ballard with conflating natural decay of the wreck with human damage. “Your ‘famous’ holes on the deck are not caused by submersibles,” he wrote. “They grow year after year.” He called Dr. Ballard unfamiliar with shipwrecks in general.

The next year, in 2006, Dr. Ballard published a lengthy retort. He referred to Mr. Nargeolet as “the author of these statements,” as if naming the Frenchman gave him undue respect.

Denying naïveté, Dr. Ballard said he knew of “no other person on Earth that has worked on as many shipwrecks in the deep sea as I have.” But his list of 57 vessels named only two that predated Titanic’s discovery. And they were not shipwrecks but lost submarines.

Recently, the two men published dueling books. Dr. Ballard’s, in 2021, as usual, made no mention of Mr. Nargeolet by name. It characterized the artifact seekers as “vultures.”

Dr. Ballard told of gathering “vivid images” meant to document human damage to the famous liner. “I wanted people to start thinking about the oceans themselves as a museum to be visited and respected, but not pillaged,” he wrote.

In 2022, Mr. Nargeolet’s book, published in France, recounted his decades of Titanic dives, explorations and artifact recoveries.

In his description of the wreck’s discovery, Mr. Nargeolet highlighted an awkward moment for Dr. Ballard that he had learned about. It happened late at night. In the control room, Mr. Michel, leader of the French team and co-leader of the expedition, was monitoring a tethered robot when, suddenly, images of wreck debris appeared on the video screen. It was the lost ship!

Dr. Ballard was off duty in his cabin, and to locate him the excited team sent the ship’s cook. The anticlimax, Mr. Nargeolet remarked, “does not prevent Bob Ballard from proclaiming himself the sole discoverer of the wreck.”

Mr. Nargeolet in his book also described Dr. Ballard as breaking an agreement over the distribution of Titanic photos, calling him a man “for whom fair play is not a strong point.” Dr. Ballard in his memoir blamed his Woods Hole boss for the breach. “The French were outraged,” he wrote, “and so was I.”

In an email, Dr. Ballard said he had a record of good relations with French oceanographers long before the Titanic’s discovery and had shared credit with them repeatedly, including in a National Geographic article. He and Mr. Nargeolet, he added, had “little in common.” Asked if he regretted casting the submariner and his peers as grave robbers, he made no comment.

“There is nothing to learn from recovering objects,” Dr. Ballard said in an email. “The Titanic belongs to the sea.”

Final Transmissions

By Saturday, June 17, the bad weather had finally let up. Mr. Nargeolet was preparing to make what would be his final dive when an email landed from Mr. Jarry. His French colleague said he had read and enjoyed Mr. Nargeolet’s book. In an interview, Mr. Jarry said he discussed Dr. Ballard in the email exchange but declined to share parts of Mr. Nargeolet’s reply concerning the American oceanographer.

The American, Mr. Jarry remarked, loved the fame of discovery while the Frenchman loved the work of artifact preservation. “He was happy doing that,” Mr. Jarry said.

Michel L’Hour, a close friend of Mr. Nargeolet’s who ran France’s underwater archaeology program, spoke to his old friend in May by video call, just before Mr. Nargeolet joined the Titan team in St. John’s, Newfoundland. He argues that the feud with Dr. Ballard played no role in Mr. Nargeolet’s risky dives and instead points to the submariner’s trust in his technical skills.

“I said, ‘Aren’t you a little scared?’” Mr. L’Hour recalled. Mr. Nargeolet said he was, but believed he could help improve the novel technology.

“He saw it like a scientist in a lab, doing tests and learning as he was going,” Mr. L’Hour said. As an accomplished diver, he added, Mr. Nargeolet knew risk management. “A risk is not a danger. A risk is something you can master.”

Mr. Vescovo, the diver who hired Mr. Nargeolet, agreed. “I think he thought that, given his knowledge and experience, if anybody could make Titan safer, it was him.”

Mr. Nargeolet’s legacy will live on, according to his former employer. RMS Titanic Inc. is planning new recoveries, despite losing his wide knowledge of the wreck site.

High on the company’s list is one of Mr. Nargeolet’s dreams — recovering the Marconi wireless telegraph famed for transmitting Titanic’s distress calls. Responding ships helped save hundreds of lives, many of them women and children in lifeboats.

Jessica Sanders, president of RMS Titanic, said the company, in honor of Mr. Nargeolet, was committed to establishing new teams that will pick up where he left off in preserving what remains of the shipwreck, now 111 years old.

The aim, she said, “is to inspire that next generation — the next him.”