The search for the missing submersible on an expedition to view the wreckage of the Titanic neared the critical 96-hour mark on 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 a 96-hour supply of breathable air when it launched Sunday morning in the North Atlantic.

That means the deadline to find and rescue the sub is roughly between 6 am (1000 GMT) and 8 am (1200 GMT) on Thursday based on estimates the US Coast Guard and company behind the expedition have provided.

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Experts say that the oxygen supply number 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.

Officials have expanded the coverage area to thousands of miles — twice the size of Connecticut and in waters 2 1/2 miles deep. Airplanes, ships, remote-operated underwater vehicles and a US Navy-owned specialized salvage system have been deployed, concentrating where unidentified noises had been heard for two days. The noises raised hopes of a possible discovery.

The Titan was reported overdue Sunday afternoon about 435 miles (700 kilometres) 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, an undersea exploration company, has been chronicling the Titanic's decay and the underwater ecosystem around it via yearly voyages since 2021.

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