Here's what we know about the five men who set out on a costly tourism expedition aboard the OceanGate vessel, named the Titan, and had been at the centre of a global search operation since Sunday.
Rush, the pilot of the submersible, is the founder and CEO of OceanGate, a private research and tourism company that has conducted more than a dozen underwater expeditions since 2010. The company was created in 2009 and had completed other dives to the Titanic wreck in the past two years.
Rush, 61, obtained his DC-8 Type/Captain's rating at the United Airlines Jet Training Institute in 1981 at age 19, making him the youngest jet transport-rated pilot in the world, according to his biography on OceanGate's website. His pilot license has taken him to Cairo; Damascus, Syria; Mumbai; London; Zurich; and Khartoum, Sudan.
He obtained a Bachelor of Science degree in aerospace engineering from Princeton University in 1984, and an MBA from the UC-Berkeley Haas School of Business in 1989. That year, Rush built a Glasair III experimental aircraft that he still flies, according to his company bio. He also finished a heavily modified Kittredge K-350 two-person submersible, in which he has conducted more than 30 dives.
In December, Rush told "CBS Sunday Morning" that he believed diving in his submersible was not particularly dangerous but that he worried about something happening that would keep him from returning to the surface.
"I mean, if you just want to be safe, don't get out of bed," he said. "Don't get in your car. Don't do anything. At some point, you're going to take some risk, and it really is a risk-reward question."
For Rush, ocean exploration was a calling. He told CBS that he dreamed of being an astronaut when he was a child but later realized he wanted to probe a different unknown.
"I wanted to be sort of the Captain Kirk," he said. "I didn't want to be the passenger in the back."
Hamish Harding is a British aviation businessman and seasoned adventurer who has set several world records for deep-sea travel and circumnavigating the Earth via airplane.
Harding, an aircraft pilot, posted on social media before the trip that he was set to travel to the Titanic wreckage on the Titan.
"This mission is likely to be the first and only manned mission to the Titanic in 2023," he wrote on Instagram.
Harding is the founder and chairman of Action Aviation. He is in his late 50s and also chairs the Middle East chapter of the Explorers Club, which describes itself as a "multidisciplinary, professional society dedicated to the advancement of field research, scientific exploration and resource conservation."
Harding was a passenger last year on the fifth human spaceflight of Blue Origin, the private space company founded by Jeff Bezos, who is the honorary chair of the Explorers Club. (Bezos owns The Washington Post.)
Harding's social media pages are filled with photos of planes, rockets, ships and shots of his adventures to destinations including the South Pole and the Mariana Trench.
Nargeolet, a 77-year-old retired French navy commander, is the director of underwater research for a media and exhibition company whose affiliate, RMS Titanic, is the exclusive steward of the wreck.
Nargeolet, nicknamed P.H., has "an unparalleled knowledge" of the wreckage, said Brandon Whited, trustee of the Titanic International Society, which traveled to the site with Nargeolet in the 1990s. Nargeolet can identify pieces of the ship's stern, which is in shambles on the ocean floor, "like no one else can," Whited said.
"He truly knows the ship and has almost an unbelievable passion for it," Whited said.
Nargeolet was born in Chamonix, France, and also lived across Africa for 13 years with his family. He served as a submarine pilot, ship captain and deep diver during his two decades in the French navy, according to his LinkedIn profile.
While working at the French marine science research institute IFREMER, Nargeolet ventured down to the Titanic for the first time in 1987 - two years after the wreckage was found. He told HarperCollins last year that he had intended to go in 1986, but the plan didn't pan out.
Nargeolet described descending in a small submarine and silence falling when the passengers arrived at the wreck to see shining bronze anchors and chains, buffed clean by the sea, the print on them saying they were made in Glasgow still clear.
"We were extremely happy, finally, to start to dive to the Titanic," he told HarperCollins in French. "And we absolutely didn't know at the time that I would return as often as I have been able to do."
Nargeolet has since led "several expeditions" to the site, completed 37 submersible dives - including with OceanGate - and overseen the recovery of 5,000 artifacts, according to a biography on his company's website.
Last year, Nargeolet published a book on the ship in French: "Dans les profondeurs du Titanic," or "In the Depths of the Titanic."
"He knew that area so well. That's what's so tragic," said Michael Findlay, former president of the Titanic International Society and a friend of Nargeolet. "He's made more dives to that location than anyone. And the fact that he's among those that are missing, it's almost too impossible to comprehend."
British Pakistani businessman Shahzada Dawood, 48, and his son, Suleman, 19, were also on the expedition, their family confirmed in a statement. They are part of one of Pakistan's most prominent business families.
Shahzada Dawood is vice chairman of the Pakistani conglomerate Engro Corp., which has a mix of industrial interests including fertilizers, textiles and food. It is a subsidiary of family-owned Dawood Hercules, fronted by his father, Hussain.
Dawood also directs the family's Dawood Foundation and is a trustee of the SETI Institute, a California-based research organization that searches for extraterrestrial life. The institute's president, Bill Diamond, said Dawood is humble by nature and fascinated by the question of whether intelligent life exists elsewhere in the universe.
When he told Diamond about the submersible trip in April, Dawood expressed excitement at what he saw as an extraordinary opportunity. He isn't usually a risk-taker, Diamond said, but he seemed intrigued by the opportunity to explore the ocean - still a relatively unknown frontier for humans.
"He said this was going to an extreme depth in a private craft, so I think he understood this was kind of at the edges of technological capabilities for us, but he didn't seem to dwell on the risk," Diamond said.
Dawood holds dual British-Pakistani citizenship and sits on a number of charitable boards, including some linked to Britain's King Charles III. He has spoken at the United Nations and frequented the World Economic Forum in Davos, Switzerland, where he shared his vision for a "sustainable future" and championed clean energy and business models that help uplift "low-income communities."
He is a fan of wildlife photography, gardening, his dog and exploring various natural habitats, his family said in the statement.
Suleman Dawood is the youngest member of the Titanic expedition. He grew up outside London in the leafy village of Long Ditton, in Surrey, with his father and mother, Christine, a professional development and life coach who has run an organic agriculture business in Pakistan's Punjab region and sits on the board of the Dawood Foundation.
Suleman is a fan of science fiction books, solving Rubik's Cubes and playing volleyball, his family said in a statement. He recently graduated from ACS International School Cobham, whose head teacher Barnaby Sandow told The Post that the school community "is deeply concerned by the news" that Suleman and his father are on board the missing submersible.
"Our thoughts are with the family, friends and loved ones, during this difficult time," Sandow said in a statement.
Suleman has a sister, Alina, and the family has a dog named Stig and a cat named Proteus. Their neighbors told the Daily Mail: "You couldn't wish for lovelier neighbors. . . . The children are so sweet and polite."