A renowned Titanic expert, a world record-holding adventurer, two members of one of Pakistan's wealthiest families and the CEO of the company leading an expedition to the world's most famous shipwreck have died aboard the Titan submersible after it imploded in the Atlantic Ocean.
The US coast guard on Thursday said there were no survivors after the catastrophic implosion deep in the North Atlantic.
The search for the submersible and its occupants - and any clues to explain what happened underwater - was ongoing on Thursday after a deep-sea robot found debris near the Titanic shipwreck.
The expedition featuring the Titan was led by OceanGate, making its third voyage to the Titanic, which struck an iceberg and sank in 1912, killing all but about 700 of the roughly 2,200 passengers and crew.
Here's what we know about the people who were on board.
Shahzada Dawood and his son Suleman
Shahzada Dawood, right, and his son Suleman. Credit: Dawood family
Mr Dawood was vice chairman of one of Pakistan's largest conglomerates, Engro Corporation, with investments in fertilisers, vehicle manufacturing, energy and digital technologies.
He was on the board of trustees for the California-based SETI Institute, which searches for extraterrestrial intelligence. He was also a member of the Global Advisory Board at the Prince's Trust International, founded by King Charles III to address youth unemployment
Condolences poured in for the father-son duo from Pakistan's Foreign Ministry, government officials, friends and community members.
Mr Dawood's interests included wildlife photography, gardening and exploring natural habitats, while his son Suleman was a fan of science fiction literature, according to a statement from the Dawood Group.
Hamish Harding
Hamish Harding was a British businessman who lived in Dubai in the United Arab Emirates.
Action Aviation, an aircraft brokering company for which Harding served as chairman, said he was one of the mission specialists who paid to go on the expedition.
Dubai-based Harding had posted on social media that he was proud to be heading to the Titanic as a "mission specialist", adding: "Due to the worst winter in Newfoundland in 40 years, this mission is likely to be the first and only manned mission to the Titanic in 2023."
Mr Harding was also on board the 2019 "One More Orbit" flight mission that set a record for the fastest circumnavigation of Earth by aircraft over both geographic poles.
The billionaire adventurer held three Guinness World Records, including the longest duration at full ocean depth by a crewed vessel. He dived to the lowest depth of the Mariana Trench with ocean explorer Victor Vescovo in March 2021.
Paul-Henri Nargeolet
A former French navy officer, he was considered a Titanic expert after making multiple trips to the wreckage across several decades.
He was the director of underwater research for E/M Group and RMS Titanic Inc. He completed 37 dives to the wreck and supervised the recovery of 5,000 artefacts, according to his company profile.
RMS Titanic, Inc, the company that owns the salvage rights to the Titanic shipwreck, said the longtime employee known as "PH" was "an iconic and inspirational leader in deep-sea exploration".
Paul-Henri Nargeolet, director of a deep ocean research project dedicated to the Titanic, poses inside the new exhibition dedicated to the sunken ship. Source: AFP / Joel Saget
He also led the first recovery expedition to the Titanic in 1987 for the French Institute for Research and Exploitation of Sea.
After retiring from the navy, he led the first recovery expedition to the Titanic in 1987 and was a leading authority on the wreck site.
In a 2020 interview with France Bleu radio, he spoke of the dangers of deep diving, saying: "I am not afraid to die, I think it will happen one day".
Stockton Rush
Mr Rush was the Titan's pilot.
He had a background in aerospace and technology but founded OceanGate Inc, a private company based in Washington, in 2009 to provide crewed submersibles for undersea researchers and explorers.
OceanGate started bringing tourists to the Titanic in 2021 as part of its effort to chronicle the slow deterioration of the wreck.
"The ocean is taking this thing, and we need to document it before it all disappears or becomes unrecognisable," Mr Rush said in 2021.
He defended the safety of his submersible in an interview with CBS News last year but said nothing was without risk.
"What I worry about most are things that will stop me from being able to get to the surface - overhangs, fish nets, entanglement hazard," he said, adding a good pilot could avoid such perils.
OceanGate CEO and co-founder Stockton Rush during a presentation on findings after an undersea exploration of the SS Andrea Doria wreckage in the Atlantic Ocean near Nantucket, Boston. Source: AP / Bill Sikes
He joined McDonnell Douglas Corp in 1984 as a flight test engineer and oversaw the development of multiple successful IP ventures during the past 20 years.
Greg Stone, a longtime ocean scientist and a friend of Rush, called him "a real pioneer" in the innovation of submersibles.
"Stockton was a risk-taker. He was smart. He was, he had a vision - he wanted to push things forward," Mr Stone said.
What's the firm behind this?
Based in Everett, Washington, OceanGate says it uses next-generation crewed submersibles and launch platforms to increase deep ocean access as far as 4,000 metres.
"OceanGate has successfully completed over 14 expeditions and over 200 dives in the Pacific, Atlantic and Gulf of Mexico," its website says.
"Following every mission, the team evaluates and updates the procedures as part as a continued commitment to evolve and ensure operational safety."
What is the vessel?
Although popularly called a submarine, in marine terminology, the "Titan" vessel carrying the five is a submersible.
While a submarine can launch itself from a port independently, a submersible goes down off a support ship.
The company was issued a serious warning about safety problems with the Titanic voyage in 2018.
More than thirty people, including industry leaders, deep-sea explorers and oceanographers, said in a letter to Mr Rush that the company's "experimental" approach and its decision to forgo a traditional assessment could lead to potentially "catastrophic" problems.
Can the families sue?
The passengers, who paid as much as $US250,000 ($A370,000) each for the journey to 3,810 metres below the surface, are believed to have signed liability waivers.
Waivers are not always ironclad, and it is not uncommon for judges to reject them if there is evidence of gross negligence or hazards that were not fully disclosed.
"If there were aspects of the design or construction of this vessel that were kept from the passengers or it was knowingly operated despite information that it was not suitable for this dive, that would absolutely go against the validity of the waiver," Texas-based personal injury lawyer and maritime law expert Matthew D Shaffer said.
OceanGate could argue it was not grossly negligent and that the waivers apply because they fully described the dangers inherent in plumbing the deepest reaches of the ocean in a submersible the size of a minivan.
