NEWFOUNDLAND - A submersible taking wealthy tourists to visit the site of the Titanic wreckage in deep waters off the coast of Canada was missing for a third day on Tuesday, as US and Canadian ships and planes swept a huge area trying to find the vessel.

One pilot and four passengers were on board the submersible that went missing on Sunday, the United States Coast Guard said, adding the vessel could stay underwater for up to 96 hours – about four days – although it was unclear if it had resurfaced but was unable to communicate.

The authorities are expanding their search into deeper waters overnight, US Coast Guard Rear Admiral John Mauger said on Tuesday, with the authorities prioritising the area where the vessel was operating.

“As we continue on with this search... we’ve been working through the night with a broad group of partners to bring all capabilities to bear looking on both the surface and now expanding to a subsurface in the area,” he told CNN in an interview.

The Coast Guard confirmed on Monday that it was searching for the vessel after the Canadian research ship MV Polar Prince lost contact with the submersible about 1hr and 45min after it began a dive about 1,450km east of Cape Cod, Massachusetts, on Sunday morning towards the Titanic’s wreckage.

“It is a remote area and it is a challenge to conduct a search in that remote area, but we are deploying all available assets to make sure that we can locate the craft and rescue the people on board,” said Rear Adm Mauger.

France’s oceanographic institute is sending its vessel Atalante, equipped with a deep-sea underwater robot, to the north Atlantic to help find the submersible, the maritime ministry said, adding that the ship should arrive in the area on Wednesday.

OceanGate Expeditions, the private company that operates the submersible, said it was “mobilising all options” to rescue those aboard the Titan.

Rear Adm Mauger said OceanGate is leading the search efforts, with aircraft from the US and Canada involved in the search, and sonar buoys deployed to help search under the surface. 

“They know that site better than anybody else,” he said on NBC.

Asked if he was optimistic that the tourists could be found alive, Mr Mauger said crews were focused on finding the people onboard and the capabilities needed to continue the search. He declined to confirm the identity of anyone on board, citing privacy issues.

The submersible disappeared in a portion of the ocean with a depth of roughly 4,000m.

Mr Mauger said it was designed to sustain its occupants for 96 hours in an emergency. This means the vessel has 96 hours of oxygen in reserve in the event of an emergency. But given that the submersible has been missing for more than 48 hours, that amount is steadily depleting.

Even if the Titan can be located, retrieving it will not be easy. That is partly because even the best divers cannot safely go more than a few hundred feet below the surface.

To recover objects off the seafloor, the US Navy uses a remote-operated vehicle. But ships that carry such a vehicle normally move no faster than about 30kmh, and the Titanic wreck lies about 595km off the coast of Newfoundland.

Mr Hamish Harding, the chair of the aviation company Action Aviation, is among those aboard the missing submersible, according to the company’s managing director, Mr Mark Butler. Mr Harding wrote on his Facebook page on Saturday that a dive had been planned for Sunday. “A weather window has just opened up,” he wrote.

Another person believed to be on board is former French navy commander Paul Henry Nargeolet, a deep diver and a submersible pilot. He is the director of underwater research for E/M Group and RMS Titanic, and is widely considered a leading authority on the wreck site, according to The Guardian. He has led several expeditions to the Titanic site.

It is possible that he was in charge of the submersible on the dive.

Mr Shahzada Dawood and his son Suleman, members of one of the most prominent business families in Pakistan, are also on board, their relatives confirmed in a statement. Mr Shahzada Dawood is vice-chairman of Engro Corp, which has businesses stretching from fertilisers to power generation.

Mr Stockton Rush, the chief executive and founder of OceanGate Expeditions, is also on board, Sky News reported. He had hoped to make the Titanic more accessible with visits to the wreckage aboard his privately owned five-person submersible.

The initial goal was to take paying guests to the site on weekly visits from May to September, coupling the trips with research efforts that allow passengers to contribute as citizen scientists.

It has also emerged that another guest, Mr David Concannon, a lawyer who also runs an exploration consultancy firm, was slated to join the trip but was forced to cancel at the last minute.

OceanGate offers 10-day expeditions to the Titanic site, providing “qualified explorers” the opportunity to join as mission specialists. They pay fees to underwrite their training and participation with the science teams exploring the legendary ship that sank in 1912.

Guests pay US$250,000 (S$335,000) to travel to the wreckage on the seabed, more than 3.2km below the ocean’s surface.

The submersible takes about two hours to descend to the Titanic. At that depth, the water pressure is hundreds of times as high as it is just below the surface.

A submersible travelling down to the Titanic faces soaring, crushing pressure during its long descent.

At the ship’s resting place, the weight of the icy ocean pressing down would be equal to a tower of solid lead overhead rising to the height of New York’s Empire State Building.

OceanGate says customers are not required to have any previous diving experience, but that there are “a few physical requirements like being able to board small boats in active seas”.

The Titanic sank in the early hours of April 15, 1912, after hitting an iceberg on its maiden voyage from England to New York, killing more than 1,500 people.

The wreckage was found in 1985, broken into two main sections, about 640km off Newfoundland in eastern Canada.

The Titanic shipwreck has garnered intense interest among researchers and treasure hunters captivated by its tragic history: the horror of the accident, the inadequacy of the lifeboats, the supposed hubris of the ship’s builders and operators, the enormous wealth of many and the poverty of others on board, and the deadly indifference of the iceberg and the sea.

The story has been immortalised in non-fiction and fiction books as well as the 1997 blockbuster movie Titanic, which reinvigorated interest in the ship.

Tourists were paying for dives by submersible in the early 2000s, and salvage crews hunted for artefacts to bring back up.

At around the same time, scientists were warning that visitors were a threat to the wreck, saying that gaping holes had opened up in the decks, walls had crumpled, and that rusticles – icicle-shaped structures of rust – were spreading all over the ship.

Researchers said the site was also littered with beer and soda bottles and the remains of salvage efforts, including weights, chains and cargo nets.

Preservationists have said the wreck should be honoured as the graveyard of those who perished. NYTIMES, BLOOMBERG, REUTERS