A French diver and a Pakistani father and son are also among the missing.
Hamish Harding, one of the five people aboard a submersible missing near the Titanic wreck, is no stranger to exploration.
Harding circumnavigated the Earth in 2019. He made a dive in 2021 to the deepest point on earth, Challenger Deep, in the Mariana Trench. And he traveled into space last year aboard Blue Origin's New Shepard.
The U.S. Coast Guard on Sunday launched an urgent search for the OceanGate Inc. tourist submersible carrying Harding and four other passengers.
"Our entire focus is on the wellbeing of the crew and every step possible is being taken to bring the five crew members back safely," OceanGate said as part of an emailed statement.
The submersible, which last pinged in a location close to the Titanic wreck, was designed with life support to sustain five crew members for 96 hours.
Here's what we know about the people onboard.
Harding, a British businessman and chairman of Action Aviation, previewed his dive to see the Titanic shipwreck in a Facebook post.
"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," he posted on Sunday. "A weather window has just opened up and we are going to attempt a dive tomorrow."
Harding, a licensed air transport pilot who holds three Guinness World Records, is an "an extraordinarily accomplished individual who has successfully undertaken challenging expeditions," Action Aviation said in a statement.
Along with his ocean dives and blasting off to space, he's helped with "the reintroduction of Cheetahs from Namibia to India" and "has been to the South Pole a number of times," the company said. Harding was inducted in 2022 as a Living Legend of Aviation.
"The team at Action Aviation are extremely proud of Hamish and we look forward to welcoming him home," the company said.
Shahzada Dawood is vice chairman of Engro Corporation Limited, a sprawling business headquartered in Karachi, Pakistan. Suleman Dawood is his son, the family said in a statement.
"Our son Shahzada Dawood and his son, Suleman, had embarked on a journey to visit the remnants of the Titanic in the Atlantic Ocean. As of now, contact has been lost with their submersible craft and there is limited information available," the Dawood family said.
The Dawoods are both British citizens, according to a colleague.
Shahzada Dawood, a husband and father of two, loves photography, gardening and exploring natural habitats.
Suleman Dawood, a university student, is passionate about science fiction literature and learning new things.
The family said they're grateful "for the concern being shown by our colleagues and friends and would like to request everyone to pray for their safety while granting the family privacy at this time. The family is well looked after and are praying to Allah for the safe return of their family members."
Paul-Henri Nargeolet, known as PH, a diver and Titanic researcher, is among the passengers. He's the director of Underwater Research at RMS Titanic Inc., according to the company.
Nargeolet has led six expeditions to the Titanic site, most recently in 2010.
A spokesperson, Mathieu Johann, told ABC News that Nargeolet "is indeed on board." Johann added that Nargeolet's wife lives in the U.S. and she "awaits him patiently."
Stockton Rush, the CEO of OceanGate, is also on board, the company confirmed.
According to OceanGate's website, Rush "oversees OceanGate's financial and engineering strategies and provides a clear vision for development of 4,000 meter (13,123 feet) and 6,000 meter (19,685 feet) capable crewed submersibles and their partner launch and recovery platforms."
Rush, who studied aerospace engineering at Princeton University and obtained an MBA from the University of California, Berkeley, "has written numerous engineering articles on crewed submersible vehicles in subsea operations," OceanGate's website said.
ABC News' Matt Foster, Miles Cohen, Mark Guarino, Emily Shapiro, Peter Charalambous, Sam Sweeney, Laryssa Demkiw, Gio Benitez and Victoria Beaule contributed to this report.