It is a grim scenario for the Indonesian submarine KRI Nanggala-402, which went missing north of Bali during a torpedo-firing drill on Wednesday.
It will run out of air by 3am tomorrow, as oxygen supplies on board last only 72 hours "in a blackout condition" from the time the submarine vanished, Indonesian navy chief Yudo Margono said yesterday.
"Hopefully, it will be found soon, when oxygen is still available," he told a news conference.
A maritime expert told The Straits Times yesterday that he is pessimistic, saying that the chances of survivors being found are slim.
The deeper the vessel sank, the greater the pressure on it and the crew trapped inside, said Mr Siswanto Rusdi, executive director of Indonesia's National Maritime Institute.
"The best-case scenario is that the crew suffered severe injuries such as burst eardrums and blood vessels, and the worst-case scenario is that they are dead," he added.
There has been no trace of the missing submarine, and no indication of an attempt by the crew to launch an emergency exit.
"If they had done so, modern warships and aircraft equipped with sonar capabilities would have been able to pick up the signal. But until now, we have yet to hear of such activity," he said.
Even if the crew were able to exit the submarine, only a few could be let out each time, he added.
However, Mr Siswanto noted that the sonar capability of rescue ships could be impeded by the very nature of submarines - stealth vessels designed to be undetectable under water.
The media has reported that the missing submarine could have sunk as deep as 700m, which Mr Siswanto said exceeded the vessel's maximum depth of 500m.
"What this means is the pressure was so great that the hull of the vessel could have been damaged. Survivors, if any, could have suffered broken bones, and other hyperbaric injuries," he added.
"The environment underwater is very challenging for both the vessel and the crew."
Family members of the crew are hopeful that their loved ones are alive, holding prayer sessions as they wait anxiously for official news.
University lecturer Berda Asmara, the wife of a crew member, sobbed as she spoke of her caring and affectionate husband of more than 13 years, Mr Guntur Ari Prasetya, Tribunnews.com reported.
"When my husband started working, he told me the risks of his work. He showed me a video of a missing Russian submarine. So, whether you want it or not, ready or not ready, you have to be ready," she said.
Mr Siswanto said the disaster was likely due to the old age of the vessel, which was ordered by the Indonesian government from Germany in 1977 and completed in 1980. It joined the Indonesian fleet in 1981.
A majority of Indonesian warships are more than two decades old, he said, adding: "Usually, submarines are retired after 20 to 25 years, but this submarine is double the age. No matter how much you try to repair it, it is no use. It is akin to asking a 70-year-old grandfather to pull a rickshaw. He can't go any faster."
The most important task now is to locate the submarine, but with strong currents in Bali waters, the vessel's position could be shifting.
Once it is located, an unmanned submarine and divers can be deployed to force open the submarine and locate the crew within.
Mr Siswanto expressed regret at the incident, saying that the Indonesian government should not have allowed such an old vessel to operate.
"It was a time bomb waiting to go off," he said.