NBC News, quoting a Coast Guard official, reported the people inside the 21-foot sub known as Titan would have sucked up all the breathable air by 7:08 AM ET Thursday -- and we are now well past that point.

As you probably know, the sub had 96 hours of emergency oxygen when the passengers embarked on their voyage Sunday to survey the Titanic wreckage at the bottom of the North Atlantic Ocean. The vessel, owned by OceanGate expeditions, vanished in the waters off Newfoundland and has not resurfaced.

Some experts believe the passengers may have tried to preserve what's left of the oxygen in an effort to save their lives.

Oceanographer Jules Jaffe told NBC ... "If they were imagining that they would run out of oxygen, the smart thing to do would be to reduce your metabolic effort and perhaps lay very still."

Yet, the Coast Guard maintains the oxygen is already gone.

Meanwhile, the desperate search for the sub continued Wednesday as rescuers rapidly expanded the area in which they were looking, which was twice the size of Connecticut.

That zone was where Canadian aircraft detected possible banging noises coming from inside the missing sub indicating apparent signs of life ... but Coast Guard officials warned the sounds were inconclusive.

"Simpsons" writer and producer Mike Reiss appeared on "TMZ Live" Wednesday and described his personal journey to view the Titanic wreck aboard the OceanGate -- and while he said he felt relaxed throughout, he was also fearful he might die down deep inside the ocean.