• Rescuers are racing to find a submersible that went missing with five people on board.

• The crew, on a tour by OceanGate Expeditions, could lose oxygen by Thursday afternoon.

• Here are three likely scenarios of what could have happened, according to experts.

An extensive search-and-rescue effort is underway after a tourist submersible carrying five people went missing Sunday morning while on a dive mission to the Titanic shipwreck.

The 23,000-pound Titan vessel, owned by OceanGate Expeditions, went off the radar about one hour and 45 minutes into its descent off the coast of Cape Cod, Massachusetts, the US Coast Guard said.

The crew — which includes a British billionaire adventurer and a father and son from Pakistan — may run out of oxygen by Thursday afternoon ET, experts estimate.

Underwater noises detected in the area raise the prospects that the passengers may still be alive, but time is running out to rescue them.

As the rescue efforts continue, here are three scenarios of what could have happened.

1. Titan may be bobbing around on the surface, waiting to be found

This would be the “best-case scenario,” Stefan B. Williams, a professor of marine robotics at the University of Sydney whose lab works with uncrewed submersibles, told Insider.

Williams said there was a possibility the device the Titan used to communicate with its support ship, the Polar Prince, somehow failed and that it had already resurfaced and was waiting to be found.

“That’s a possibility, but presumably, at the end of the dive, the vessel would’ve surfaced as expected,” Williams said. “That’s seeming increasingly unlikely as time goes past.”

Vessels like the Titan are usually designed to either drop weights or inflate floaters automatically to bring them back to the surface if something goes wrong, Williams added.

The retired Navy Rear Adm. Chris Parry told Sky News that if the vessel had come to the surface, it “would have been found by now.”

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The dive of the Titan submersible usually takes about eight hours, including the descent and ascent, according to OceanGate’s website.

The US Coast Guard has already searched an area “about the size of Connecticut” but has had no luck, CNN reported.

2. It has suffered a ‘catastrophic implosion’

If the submersible has not bobbed back up to the surface, there’s likely been a “catastrophic failure,” Williams said.

That could be due to a leak or a power failure. There’s also a chance a small fire from an electrical short circuit may have compromised the vehicle’s electronic systems, which are used for navigation and control of the vessel, Williams said in a blog post about the submersible.

The worst-case scenario is that the pressure hull was breached, leading to a “catastrophic implosion,” Williams said.

“It would happen quite quickly, and there would be little chance of surviving,” he added.

3. It has become tangled in the wreckage of the Titanic

Two experts said that another likely scenario was that the Titan could still be intact and its passengers still alive, but the vessels may have become stuck near the bottom of the ocean. It may, for instance, have gotten tangled in the wreckage of the Titanic, which lies at about 12,500 feet underwater.

The Titan, which is powered by electric thrusters, can carry five people to a depth of 13,123 feet, according to the OceanGate website.

Frank Owen, a retired Royal Australian Navy official and a project director for submarine escape and rescue, told The Guardian that the wreckage on the ocean floor “is surrounded by debris from the disaster more than a century ago.”

“There are parts of it all over the place. It’s dangerous,” he said.

This prospect is unlikely, but not impossible, Williams told Insider.

The US Coast Guard confirmed on Wednesday they picked up suspicious underwater sounds using sonobuoys, sensitive microphones strapped to buoys dropped from planes flying over the search area.

Rolling Stone previously reported that searchers on a Canadian aircraft detected “banging” at 30-minute intervals coming from the area where the submersible went missing.

Experts will be analyzing the noise to determine whether it can be traced back to its source, said Williams.

“It sort of opens the possibility that they’re still alive and gives you some hope, but of course, we’re closing in on the end of their window of the available air,” Williams said Wednesday morning.

“So even if they are alive and they can locate the submarine, they still have a fairly difficult task of trying to get it back up to the surface,” said Williams.

A seabed rescue is very difficult

In any case, a rescue from the ocean floor would be very difficult, Alistair Greig, a professor of marine engineering at University College London, told The Guardian.

“There are very few vessels that can get that deep, and certainly not divers,” he said.

Williams added that it may be difficult for the rescue team to even spot the submersible ship from the surface, as its sonars wouldn’t be able to pick it out from the debris and uneven terrain on the seabed.

Whatever the circumstances of the Titan’s disappearance, there is no doubt that if the vessel is still intact and its passengers are still alive, those trapped inside are trying to survive in a “pretty tough environment,” Williams told Insider.

Without life-support systems, the submarine could become very cold — the water temperature outside is near freezing, Williams said. Passengers also typically take only a sandwich and a bottle of water on the submersible, and there are no onboard toilets, David Pogue, a science journalist who was on the submersible last year, said in an interview with NewsNation.

David Gallo, a senior advisor for strategic initiatives at RMS Titanic, told CNN that dwindling oxygen levels and fighting the cold were the primary concerns for the passengers’ safety at this point.

“The water is very deep — 2 miles-plus,” Gallo said. “It’s like a visit to another planet. It’s not what people think it is. It is a sunless, cold environment and high pressure.”

Read the original article on Insider