The family of a French explorer who died in a submersible implosion has filed a more than $50 million lawsuit, saying the crew experienced “terror and mental anguish” before the disaster and accusing the sub’s operator of gross negligence.

Paul-Henri Nargeolet was among five people who died when the Titan submersible imploded during a voyage to the famed Titanic wreck site in the North Atlantic in June 2023. No one survived the trip aboard the experimental submersible owned by OceanGate, a company in Washington state that has since suspended operations.

Known as “Mr. Titanic,” Nargeolet participated in 37 dives to the Titanic site, the most of any diver in the world, according to the lawsuit. He was regarded as one of the world’s most knowledgeable people about the famous wreck. Attorneys for his estate said in an emailed statement that the “doomed submersible” had a “troubled history,” and that OceanGate failed to disclose key facts about the vessel and its durability.

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According to the lawsuit, the Titan “dropped weights” about 90 minutes into its dive, indicating the team had aborted or attempted to abort the dive.

“While the exact cause of failure may never be determined, experts agree that the Titan’s crew would have realized exactly what was happening,” the lawsuit states. “Common sense dictates that the crew were well aware they were going to die, before dying.”

The lawsuit goes on to say: “The crew may well have heard the carbon fiber’s crackling noise grow more intense as the weight of the water pressed on Titan’s hull. The crew lost communications and perhaps power as well. By experts’ reckoning, they would have continued to descend, in full knowledge of the vessel’s irreversible failures, experiencing terror and mental anguish prior to the Titan ultimately imploding.”

A spokesperson for OceanGate declined to comment on the lawsuit, which was filed Tuesday in King County, Washington. The defendants must respond to the complaint in the coming weeks, court papers state. The lawsuit describes Nargeolet as an employee of OceanGate and a crew member on the Titan.

The suit also criticizes Titan’s “hip, contemporary, wireless electronics system, and states that none of the controller, controls or gauges would work without a constant source of power and a wireless signal.

Though OceanGate designated Nargeolet as a member of the crew, “many of the particulars about the vessel’s flaws and shortcomings were not disclosed and were purposely concealed,” the attorneys, the Buzbee Law Firm of Houston, Texas, said in their statement.

Tony Buzbee, one of the attorneys on the case, said one of the suit’s goals is to “get answers for the family as to exactly how this happened, who all were involved, and how those involved could allow this to happen.”

Concerns were raised in the aftermath of the disaster about whether the Titan was doomed due to its unconventional design and its creator’s refusal to submit to independent checks that are standard in the industry. Its implosion also raised questions about the viability and future of private deep-sea exploration.

The U.S. Coast Guard quickly convened a high-level investigation, which is ongoing. A key public hearing that is part of the investigation is scheduled to take place in September.

The Titan made its last dive on June 18, 2023, a Sunday morning, and lost contact with its support vessel about two hours later. After a search and rescue mission that drew attention around the world, the wreckage of the Titan was found on the ocean floor about 984 feet (300 meters) off the bow of the Titanic, about 435 miles (700 kilometers) south of St. John’s, Newfoundland.

OceanGate CEO and cofounder Stockton Rush was operating the Titan when it imploded. The lawsuit describes Rush as “an eccentric and self-styled ‘innovator’ in the deep-sea diving industry” and names his estate as one of the defendants.

In addition to Rush and Nargeolet, the implosion killed British adventurer Hamish Harding and two members of a prominent Pakistani family, Shahzada Dawood and his son Suleman Dawood.

The company that owns the salvage rights to the Titanic is in the midst of its first voyage to the wreckage site in years. Last month, RMS Titanic Inc., a Georgia-based firm, launched its first expedition to the site since 2010 from Providence, Rhode Island.

Nargeolet was director of underwater research for RMS Titanic. He was part of an expedition to visit the Titanic site in 1987, shortly after its location was discovered, and had supervised the salvage of innumerable Titanic artifacts, the lawsuit states. His estate’s attorneys described him as a seasoned veteran of underwater exploration who would not have participated in the Titan expedition if the company had been more transparent.

The lawsuit blames the implosion on the “persistent carelessness, recklessness and negligence” of Oceangate, Rush and others.

“Decedent Nargeolet may have died doing what he loved to do, but his death - and the deaths of the other Titan crew members - was wrongful,” the lawsuit states.