The US Coast Guard is looking into "banging noises" heard near the Titanic shipwreck in the Atlantic Ocean.
Officials think the five people on board a missing sub only have enough oxygen to last until tomorrow morning.
Efforts to find them are continuing off the coast of Canada today.
Captain David Marquet is a retired Submarine Commander in the US Army.
He says they'll need specialist ships with cables to pull them back to the surface.
"You need to be doing that now, so that when you find it you're not starting freshing, saying 'oh let's look around to see if we can find a ship that's got a cable'.
"It sort of almost sounds to me like they've admitted that the chances are really, really, really slim on this - so slim as probably not even worth going to the next step."
