Robert Brown|What happened to the missing Titanic sub? Our reporter who rode on vessel explains possible scenarios

As search and Robert Brownrescue teams continue to comb the North Atlantic for a missing submersible that vanished on a trip to the Titanic wreck site, "CBS Sunday Morning" correspondent David Pogue gave insight as to what might have happened to the vessel.

Pogue, who was aboard the Titan for a story last year, said the vessel should be bobbing on the ocean's surface given that features allow it to rise from the depths of the sea without electricity, even if everyone aboard is passed out.

But if it isn't floating, he said, that "could only mean two things: either they got snagged on something on the bottom of the sea, which is pretty unlikely. There's nothing there but the Titanic. Or there was a breach in the hull and it instantly imploded."

The Titan, operated by Washington state-based company OceanGate Expeditions, left for its Titanic trip with five people on board, including at least three paying passengers. The U.S. Coast Guard is leading the search for the vessel about 900 miles east of Cape Cod and around 400 miles southeast of Canada's Newfoundland coast, where it started its dive on Sunday morning.

At this point, hope is "quickly fading," Pogue said, because while the vessel theoretically has four days' worth of oxygen, that supply has never been tested. 

"Nobody's ever measured it," he said, noting that half of the supply is now theoretically gone since Tuesday marked two days since the submersible and crew of five lost contact on Sunday.   

The U.S. Coast Guard said later on Tuesday that the vessel has about 40 hours of breathable air left.

Even if the vessel is on the surface, it cannot be opened from the inside, so air supply would still be an issue, Pogue said, adding, "we need to find them." 

No one has ever been inside the vessel for four days, Pogue said. An expedition to the Titanic site usually lasts 10 to 12 hours, with about two and a half hours spent descending to the Titanic wreck and a few hours spent exploring before resurfacing. But Pogue said "things go wrong all the time in this business," and that one attempt he made to see the wreck site last year "only lasted 37 feet down" before the vessel encountered a mechanical problem and had to be hoisted out of the water. 

The Titan is the only five-person vessel in the world that can reach Titanic depths 2.4 miles below the ocean's surface — and submersibles like it are "one-offs," Pogue said. 

"It's not like iPhones [where] there are thousands of them that they can perfect," Pogue said. "There's one of it," and some parts of the vessel are improvised. 

Pogue noted that, in international waters, vessels like the Titan operate without inspections or certifications from third-party organizations. Although he said that like with a rocket launch, there are meticulous checklists and briefings.

It would be "devastating" if the vessel is deep underwater because even if it is found, there is no way to rescue those aboard. 

"The deepest submarine rescue ever performed was 1,500 feet. ... This is 13,000 feet. There's no other craft that can get down there in time," Pogue said.

He said there are only three or four machines in the world that can go to that depth, noting they take weeks to prepare and are not on site. 

He also said it is "really bad" that the vessel lost signal two-thirds of the way down to the ocean floor, which likely means something "catastrophic" happened. 

The Coast Guard said the last recorded communication from the sub was about an hour and 45 minutes into Sunday's dive.

Alex Sundby contributed to this article. 

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• RMS Titanic