CHICAGO (NewsNation) — As time runs out to locate and rescue the missing Titanic tour submersible in the North Atlantic Ocean, more deep-sea experts explore the challenges the passengers could be facing on board and at such great depths.

The five people aboard the Titan sub have less than 24 hours of oxygen supply left inside the vessel, according to the U.S. Coast Guard. And, on Tuesday, a Canadian aircraft searching for the submersible detected “underwater noise” from the vicinity of the location the crew was touring the wreck site of the Titanic ocean liner.

The crew searching for the sub heard sounds every 30 minutes and again four hours later after additional sonar devices were deployed, The Independent reported. However, after deploying an underwater robot to investigate, the Coast Guard said they didn’t find anything.

But retired U.S. Navy Capt. David Marquet said there’s still time and that the “man-made banging sound, underwater noise” is potentially “hopeful news,” and signals there could still be life.

“Banging on the side of the ship would be something that I’d be doing if I were trapped inside the submersible,” Marquet said. “You want to make a man-made sound.”

At this point in the search, crews are under pressure as the oxygen supply is running out and the assumption remains that all the passengers are still alive.

However, the question remains: Is survival even possible at this point?

“Yes, it’s going to be really, really tight,” Marquet said. “There’s a probability that whatever caused the original problem an hour and 45 minutes into the original dive back on Sunday, incapacitated the crew right then and there.”

But Marquet has never been at this depth before, so he has personally not experienced what being that deep could do to the human body. In fact, government agencies don’t operate that low and only a select number of people have ever ventured to that great of depth.

In his experience, operating on submarines, he said humans don’t normally feel the pressure changes like they would on an airplane. He explained it just feels normal.

The submersible should remain the same. However, if for some reason the submersible did have any physical damage to it, the passengers would have no chance at survival due to the immense pressure of the ocean.

What the passengers will be feeling, Marquet said, is their breathing patterns. As the oxygen levels go down, it will make it harder and harder for them to breathe — ultimately suffocating them.

But the depleting oxygen levels are not the only problem they face. While they are bleeding the oxygen in from the scuba tanks, they’re also exhaling carbon dioxide, which is poisonous to humans.

“What’s happening is, by now, they’re starting to get headaches, you get nausea, you start to get confused. And eventually, you poison yourself,” he explained. “So you want to remain calm, breathe slowly. Relax. Don’t use any air.”

Normally, the vessel would have the capability to recycle the air: removing the carbon dioxide while it adds oxygen. But as time goes on, carbon dioxide will begin to build as the submersible loses its ability to scrub enough of the poisonous gas, The Sydney Morning Herald reported.

If the vessel lost power, the passengers could suffocate faster even if there is still oxygen remaining due to the increase in carbon dioxide and lack of ability to filter it out, the report said.

“The rising level of carbon dioxide is what kills people first when they’re in an airtight environment, not the level of oxygen,” Dr. Dale Molé, the former director of undersea medicine and radiation health for the U.S. Navy, told The Daily Mail.

The Independent reported that Molé published a paper on the challenges crews trapped in sunken ships or vessels face last month.

Another concern on top of the carbon dioxide would be if the vessel’s batteries run out, causing the heaters to cut out. The passengers inside would likely become hypothermic and the situation would become fatal, the report said.