The family of a French explorer who died in the Titan submersible implosion have filed a lawsuit seeking more than $US50 million ($76 million), saying those aboard experienced “terror and mental anguish” and accusing the sub’s operator of gross negligence.

Paul-Henri Nargeolet was one of 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. It was owned by OceanGate, a company in Washington state that has since suspended operations.

Known as “Mr Titanic”, Nargeolet had visited the Titanic site many times and was regarded as one of the world’s most knowledgeable people about the famous wreck.

Lawyers for his estate said in an emailed statement that the “doomed submersible” had a “troubled history”, and OceanGate had failed to disclose key facts about the vessel and its durability.

“The lawsuit further alleges that even though Nargeolet had been designated by OceanGate to be a member of the crew of the vessel, many of the particulars about the vessel’s flaws and shortcomings were not disclosed and were purposely concealed,” the lawyers, the Buzbee Law Firm of Houston, Texas, said on Thursday (US time).

A spokesperson for OceanGate declined to comment on the lawsuit, which was filed on Tuesday in King County, Washington.

Paul-Henry Nargeolet (centre) died on the Titan, along with Shahzada and Suleman Dawood, Stockton Rush, and Hamish Harding. Photo: AAP

Tony Buzbee, one of the lawyer on the case, said a goals of the lawsuit was 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”.

There were concerns 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 US Coast Guard quickly convened a high-level investigation, which is still ongoing. A key public hearing that is part of the investigation is scheduled to for September.

The Titan made its last dive on June 18, 2023, a Sunday morning. It lost contact with its support vessel about two hours later.

After a search and rescue mission that drew global attention, the wreckage of the Titan was found on the ocean floor about 300 metres off the bow of the Titanic, about 700 kilometres south of St Johns, Newfoundland.

OceanGate CEO and cofounder Stockton Rush was operating the Titan when it imploded. As well as Rush and Nargeolet, the implosion killed British adventurer Hamish Harding and two members of a prominent Pakistani family, Shahzada Dawood and his 19-year-old 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, US business RMS Titanic Inc launched its first expedition to the site since 2010 from Providence, Rhode Island.

Nargeolet was director of underwater research for RMS Titanic. His estate’s lawyers 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.

-with AAP