Audio By Carbonatix
The US Coast Guard commander leading the search for the missing Titanic sub tells the BBC the crew may have around 20 hours of oxygen left.
Rear Admiral John Mauger says it's a very complex and difficult mission, with people working round the clock.
Earlier, the US Coast Guard said a noise heard in the search had given them "a target, a focus".
Mauger says they are working closely with leading acoustic experts to make sure they understand what that noise signature is.
More vessels are being sent to the search site, off the North American coast.
Contact with the sub - which has five people on board - was lost on Sunday as it made a 3,800m descent to the Titanic wreck.
Watch BBC News live coverage by clicking the play button at the top of this page.
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