BOSTON (AP) — A renowned Titanic expert, a world-record holding adventurer and two members of one of Pakistan’s wealthiest families are facing critical danger aboard a small submersible that went missing while descending to the world’s most famous shipwreck.

The submersible Titan was reported overdue Sunday night about 435 miles (700 kilometres) south of St John’s, Newfoundland, according to Canada’s Joint Rescue Coordination Center, spurring a desperate international rescue effort. As of Tuesday morning about 10,000 square miles (25,900 square kilometers) of the Atlantic Ocean had been searched, according to the US Coast Guard.

Rescuers were racing against the clock because the oxygen supply could run out by approximately 6 am Thursday.

The expedition 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.

The five people on the vessel include:

A British businessman, Hamish Harding lives in Dubai in the United Arab Emirates. Action Aviation, an aircraft brokering company for which Harding serves as chairman, said he was one the mission specialists, who paid to go on the expedition.

Harding is a billionaire adventurer who holds three Guinness World Records, including the longest duration at full ocean depth by a crewed vessel. In March 2021, he and ocean explorer Victor Vescovo dived to the lowest depth of the Mariana Trench. In June 2022, he went into space on Blue Origin’s New Shepard rocket.

“Both the Harding family and the team at Action Aviation are very grateful for all the kind messages of concern and support from our friends and colleagues,” the company said in a statement.

In a Facebook post Saturday, Harding said he was “proud” to be part of the mission given that the weather might make another trip to the wreck unlikely this year.

“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. “A weather window has just opened up and we are going to attempt a dive (Sunday).”

Harding was “looking forward to conducting research” at the Titanic site, said Richard Garriott de Cayeux, the president of The Explorers Club, a group to which Harding belonged.

“We all join in the fervent hope that the submersible is located as quickly as possible,” he said in a statement.

Father-and-son Shahzada and Suleman Dawood are members of one of Pakistan’s most prominent families. Their family said in a statement that they were both aboard the vessel.

“We are very 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 statement said. “The family is well looked after and are praying to Allah for the safe return of their family members.”

Their firm, Dawood Hercules Corporation, based in Karachi, is involved in agriculture, petrochemicals and telecommunication infrastructure

Shahzada Dawood also is on the board of trustees for the California-based SETI Institute that searches for extraterrestrial intelligence. The Dawoods live in the UK, according to SETI.

Shahzada Dawood is also a member of the Global Advisory Board at the Prince’s Trust International, founded by Britain’s King Charles III to address youth unemployment.

He has degrees from the University from Buckingham in the United Kingdon and Philadelphia University in the US.

Paul-Henry Nargeolet is a former French navy officer who is considered a Titanic expert after making multiple trips to the wreckage over several decades.

David Gallo, a senior adviser for strategic initiatives and special projects at RMS Titanic, said in an interview with CNN that Nargeolet was on board.

He is director of underwater research for E/M Group and RMS Titanic Inc, has completed 37 dives to the wreck and supervised the recovery of 5,000 artifacts, according to his company profile.