Five people in a submersible missing in the North Atlantic Ocean will be battling harsh conditions, if alive.

Sonar's picked up banging underwater, as searchers race against time to find the sub before its oxygen runs out.

CNN's reporting an internal US Government memo states it's unclear exactly when the noises were heard, or for how long.

Retired Australian Navy commodore, Peter Scott, says whether they have power will be a major factor.

"Breathable atmosphere will be a challenge, and cold will absolutely be another challenge for them."

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