The family of a French explorer who died in a submarine explosion has filed a lawsuit for more than $50 million, claiming the crew suffered from "terror and psychological pain" before the tragedy and accused the submarine operator of gross negligence.

Paul-Henry Nargeolet was among five people who died when the Titan submarine exploded while on a trip to the famous Titanic wreck site in the North Atlantic in June 2023. No one survived the voyage aboard the experimental submarine owned by OceanGate, a company in Washington state that has since halted its operations.

According to the lawsuit, Nargeolet, known as "Mr. Titanic," had participated in 37 dives to the Titanic site, the highest number of any diver in the world. He was seen as one of the most knowledgeable individuals about the famous wreck in the world.

His estate's lawyers stated in an emailed statement that the "ill-fated submarine" has a "troubled history," and that OceanGate failed to disclose key facts about the vessel and its integrity.

According to the lawsuit, "Titan dropped weight" approximately 90 minutes into the dive, indicating that the crew aborted the dive or attempted to do so.

While the exact cause of the failure may never be determined, experts agree that the Titan crew would have been fully aware of what was happening, "common sense indicates that the crew was fully aware that they were going to die, before they died."

The lawsuit further states: "The crew may have heard the sound of carbon fiber cracking increase in intensity with the pressure of the water weight on the Titan's structure. The crew lost communications and possibly power as well. According to expert calculations, they continued to descend, fully aware of the irreversible failure of the vessel, and suffered from terror and mental anguish before the Titan ultimately exploded."

A spokesman for OceanGate declined to comment on the lawsuit filed Tuesday in King County, Washington. The lawsuit describes Nargeolet as an employee of OceanGate and a crew member aboard the Titan.

The lawsuit also criticizes the "modern wireless electronics system of the Titan," stating that none of the controls or instruments would function without a steady power source and wireless signal.

The lawsuit also claims that although OceanGate appointed Nargeolet to be a crew member of the vessel, many details regarding defects and deficiencies of the vessel were not disclosed and were deliberately concealed, according to lawyers at Buzbee Law Firm in Houston, Texas.

Tony Buzbee, one of the lawyers in the case, stated that one of the goals of the lawsuit is to "get answers for the family about how this happened exactly, who all the parties involved are, and how the parties allowed this to happen."

Concerns have been raised following the disaster about whether the Titan's fate was doomed due to its unconventional design and its maker's refusal to undergo standard independent industry inspections. Its explosion also raised questions about the viability and future of private deep-sea exploration.

The U.S. Coast Guard rushed to conduct a high-level investigation, which is ongoing. A major public hearing that forms part of the investigation is scheduled for September.

The Titan conducted its last dive on June 18, 2023, on the morning of Sunday, and lost contact with its support ship after about two hours. Following a globally attention-grabbing search and rescue mission, the wreck of the Titan was found on the ocean floor approximately 984 feet (300 meters) from the bow of the Titanic, about 435 miles (700 kilometers) south of St. John's, Newfoundland.

The CEO and co-founder of OceanGate, Stockton Rush, was piloting the Titan when it exploded. In addition to Rush and Nargeolet, the explosion also killed British adventurer Hamish Harding and two members of a prominent Pakistani family, Shahzada Dawood and his son Suleman Dawood.

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

Nargeolet was the director of underwater research for RMS Titanic. The lawsuit noted that one of the expeditions Nargeolet undertook was the first visit to the Titanic in 1987, shortly after its discovery. His estate's lawyers described him as a veteran in underwater exploration who would not have participated in the Titan trip had the company been more transparent.

The lawsuit blames the explosion on "ongoing negligence, recklessness, and carelessness" by OceanGate, Rush, and others.

The lawsuit states that "the deceased Nargeolet may have died doing what he loved to do, but his death - and the deaths of the other Titan crew members - were unlawful."