The amount of oxygen on the missing submersible with five people on board is becoming a vital issue, some experts say, as more advanced equipment is rushed to the North Atlantic Ocean in a complex international search operation now at its most critical juncture, writes CNN.

The Titan submersible begins each trip to explore the wreckage of the Titanic with an estimated 96 hours of life support and has been missing since Sunday morning, setting up Thursday morning as a key target for finding the vessel and those on board.

Banging noises detected on Tuesday and Wednesday from underneath the water in the massive search area have provided hope for survivors - but it’s like finding a needle in a haystack and time is of the essence, experts say. Inside the 21-foot submersible, with rudimentary controls and no room for its passengers to stretch out, the crew would have had “limited rations” of food and water, officials have said.

“We have to remain optimistic and hopeful”, - Capt. Jamie Frederick, the response coordinator for the First Coast Guard District said during a news conference Wednesday. As aircraft perform searches from above and remotely operated vehicles probe underwater, the number of assets in the search operation is expected to double in the next day or two, Frederick said Wednesday.

The search area for the missing submersible stretches about two times the size of Connecticut on the surface and goes down as deep as 2-and-a-half miles, according to Frederick. “It’s going to be almost impossible. We need a miracle - but miracles do happen”, - oceanographer and water search expert David Gallo told CNN.