The search for the missing submersible on an expedition to view the wreckage of the Titanic neared the critical 96-hour mark Thursday when breathable air is expected to run out, reaching a vital moment in the intense effort to save the five people aboard.

The Titan submersible was estimated to have about a four-day supply of breathable air when it launched Sunday morning in the North Atlantic. That puts the deadline to find and rescue the sub at roughly between 6 am EDT (1000 GMT) and 8 am EDT (1200 GMT), based on information the US Coast Guard and company behind the expedition have provided.

Experts emphasised that is an imprecise estimate and could be extended if passengers have taken measures to conserve breathable air. And it’s not known if they survived since the sub disappeared Sunday morning.

Rescuers have rushed more ships and vessels to the site of the disappearance — and the US Coast Guard said Thursday that an undersea robot sent by a Canadian ship had reached the sea floor and begun looking for the sub.

Authorities are hoping underwater sounds might help narrow their search, whose coverage area has been expanded to thousands of miles — twice the size of Connecticut and in waters 2 1/2 miles (4 kilometers) deep.

The Titan was reported overdue Sunday afternoon about 435 miles (700 kilometers) south of St John’s, Newfoundland, as it was on its way to where the iconic ocean liner sank more than a century ago. OceanGate Expeditions, which is leading the trip, has been chronicling the Titanic’s decay and the underwater ecosystem around it via yearly voyages since 2021.

By Thursday morning, hope was running out that anyone on board the vessel would be found alive.

Many obstacles still remain: from pinpointing the vessel’s location, to reaching it with rescue equipment, to bringing it to the surface — assuming it’s still intact. And all that has to happen before the passengers’ oxygen supply runs out.

Dr Rob Larter, a marine geophysicist with the British Antarctic Survey, emphasised the difficulty of even finding something the size of the sub — which is about 22 feet (6.5 meters) long and 9 feet (nearly 3 meters) high.

“You’re talking about totally dark environments,” in which an object several dozen feet away can be missed, he said. “It’s just a needle in a haystack situation unless you’ve got a pretty precise location.”

The area of the North Atlantic where the Titan vanished Sunday is also prone to fog and stormy conditions, making it an extremely challenging environment to conduct a search-and-rescue mission, said Donald Murphy, an oceanographer who served as chief scientist of the Coast Guard’s International Ice Patrol. The passengers are also facing temperatures just above freezing.

Meanwhile, newly uncovered allegations suggest there had been significant warnings made about vessel safety during the submersible’s development.

Broadcasters around the world started newscasts at the critical hour Thursday with news of the submersible. The Saudi-owned satellite channel Al Arabiya showed a clock on air counting down to their estimate of when the air could potentially run out.

Captain Jamie Frederick of the First Coast Guard District said a day earlier that authorities were still holding out hope of saving the five passengers onboard.

Lost aboard the vessel is pilot Stockton Rush, the CEO of OceanGate. His passengers are: British adventurer Hamish Harding; Pakistani businessman Shahzada Dawood and his son Suleman; and French explorer and Titanic expert Paul-Henry Nargeolet.

At least 46 people successfully traveled on OceanGate’s submersible to the Titanic wreck site in 2021 and 2022, according to letters the company filed with a US District Court in Norfolk, Virginia, that oversees matters involving the Titanic shipwreck.