Ten ships and several remote subs have joined the hunt for the missing sub that was off to view the wreckage of the Titanic on Monday.
It lost signal with its mothership and more than three days later, its location remains unknown,
On board are British adventurer Hamish Harding; Shahzada and Suleman Dawood, a father and his 19-year-old son from a prominent Pakistani business family; Titanic expert and former French Navy diver Paul-Henry Nargeolet and Stockton Rush, the CEO of OceanGate, the company that operates the sub.
A French deep-sea robot has been deployed to the area where banging sounds have been heard, but whether the noises are signs of life is inconclusive.
The vessel can only be opened from the outside and is likely 4km deep, where temperatures are at hypothermic levels and the water pressure is intense.
A retired US Navy submarine captain has told CNN that if alive, the people on board the Titan will be very uncomfortable.
Captain David Marquet said they will be freezing cold from the waters around the vessel.
They will see their breath; there is likely to be frost on the inside of the submarine and they would be huddling together to conserve body heat.
Experts also say the cold environment will depress body functions and decrease the air needed.
A friend of one of the men on board, Chris Brown, pulled out of the trip due to safety concerns.
He said OceanGate did not want to become certified for one trip let alone multiple. He also raised concern about the materials used, saying they were "off the shelf".
Will Kohnen from the Marine Technology Society told CNN there are 10 submarines in the world that can go to 3.5km and deeper. All of them are certified except the OceanGate submersible.
A German businessman who was one of OceanGate's first customers for diving to the Titanic said it was like a "kamikaze operation".
Arthur Loibl told AP people had to be a little crazy to do it.
