There are five people onboard, including three wealthy tourists, and the sub's oxygen supply is now just 40 hours, giving rescue crews until Thursday morning to find it before it runs out.
But even if they locate the missing vessel in time, there is no certainty that it can safely be brought to surface.
At a press conference at the US Coast Guard Station in Boston – which is coordinating the search and rescue effort – First District Response Coordinator Capt. Jamie Frederick conceded on Tuesday that rescue was not guaranteed.