• On Thursday, OceanGate Expeditions said that the five passengers aboard the Titan sub are dead.
• The announcement was made after rescue officials found debris linked to the vessel.
• Experts told Insider about three scenarios that may have played out as the vessel was lost, and in its final moments.

Debris from a tourist submersible headed to the Titanic shipwreck was discovered on Thursday, and the vessel's five passengers likely faced a grim final moment.

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. 

On Thursday, after rescue officials linked debris to the missing vessel, OceanGate announced that the passengers aboard the ship are believed to be dead.

The crew included the CEO of OceanGate expeditions, a British billionaire adventurer, a French diver and explorer, and a father and son from Pakistan.

Here were the three scenarios experts believed could have happened when the vessel lost contact.

1. Titan has suffered a 'catastrophic implosion'

There has likely been some type of "catastrophic failure" to the Titan sub, Stefan Williams, a professor of marine robotics at the University of Sydney whose lab works with uncrewed submersibles, told Insider.

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. 

A submarine pilot hired to assess the Titanic submersible warned in 2018 that its hull monitoring system would only detect failure "often milliseconds before an implosion."

Coast Guard officials said Thursday they're still investigating what happened before the implosion.

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

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

One scenario was that the Titan could have still been intact and its passengers alive, but the vessel may have become stuck near the bottom of the ocean. It could, for instance, have gotten tangled in the wreckage of the Titanic, which lies at about 12,500 feet underwater. 

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 was unlikely, but not impossible, Williams told Insider.

3. Titan may be bobbing around on the surface

This would have been the "best-case scenario," Williams told Insider Tuesday.

He said there was a possibility the Titan had resurfaced and was waiting to be found. "That's seeming increasingly unlikely as time goes past," he added. The retired Navy Rear Adm. Chris Parry told Sky News Tuesday that if the vessel had come to the surface, it "would have been found by now."

And by Thursday, officials said that the debris from the vessel was found some 1,600 feet away from the Titanic shipwreck site — ruling out that possibility.

Underwater banging provided hope the passengers are still alive — but experts were divided

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 analyzed 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 submarine search and rescue expert said the sounds could suggest the vessel is closer to the surface than initially feared.

Frank Owen, a submarine search-and-rescue expert told the BBC his "confidence went up by an order of magnitude" when he heard the reports.

On Thursday, the Coast Guard said there "doesn't appear to be any connection between the noises and the location on the sea floor."

Conditions in the sub would be very dire

Had the teams been able to locate the craft, they would still have needed to come up with a strategy to pull it up from such depths, which has never been done before.

"There are very few vessels that can get that deep, and certainly not divers," Alistair Greig, a professor of marine engineering at University College London, told The Guardian.

Experts had previously estimated the crew's oxygen reserves would be running out around Thursday morning. Still, the US Coast Guard told Insider Thursday the search will continue at least through the end of the day.

"We have to remain optimistic and hopeful," Capt. Jamie Frederick, the response coordinator for the First Coast Guard District said during a news conference Wednesday, per CNN.

Matthew Schanck, search and rescue expert and founder of MarSAR International, told Insider members of the crew included a seasoned diver — PH Nargeolet — who might have known how to best conserve oxygen until the very last moment, so the estimate that oxygen could run out by midday might not be accurate, Schanck said.

"We'll always continue searching until we are absolutely certain that there is going to be no chance of survival," he said.

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