The daughter of one of five people inside the submersible missing near the wreck of the Titanic said on Thursday she continued to hope they will be rescued but she is comforted by the knowledge that her father, oceanographer Paul-Henri Nargeolet, is in the place he loved most.

Sidonie Nargeolet told Reuters she was living with "a lot of stress, very mixed emotions" as the search for the submersible in the depths of the Atlantic Ocean entered a critical phase, with air expected to run out for the five people aboard.

"At times, I have a lot of hope, I am fine, I believe and have a lot of hope. But in others, hope goes away and it is hard to endure, and as more time goes by it gets harder," Nargeolet, 39, told Reuters in the town of La Massana in Andorra where she lives.

"I really hope they will find them and safe. I think we have to trust what they are doing and be confident," she said.

Nargeolet said her 77-year-old father had been near the Titanic wreck 30 times, the first in 1987. On one trip in another vessel operated by US-based OceanGate Expeditions he told her that he was not sure he would return safely, although he eventually did.

Digital art of the RMS Titanic sailing through icebergs. (credit: Wikimedia Commons)
Digital art of the RMS Titanic sailing through icebergs. (credit: Wikimedia Commons)

She said the Titanic has been his life's "greatest passion and obsession"

She said she was realistic about time running out.

"He knows how to manage the situation, what to do, what to tell others to do, how to breath, stay relaxed. He controls very well his emotions and transmits calm to others. I am totally confident of this," she said.

"What worries me is that they are not being found because there will be a moment in which they will run out of oxygen," Nargeolet said.

"Sometimes I don't check (the news) because I don't want to hear them saying that they now have very low oxygen. I prefer to listen to positive things, to hope, that they will continue looking for them," she said.

"If they are not found, it will be very sad for us because we will not see him again. What he liked the most was to be in a submarine, (near) the Titanic. He is where he really loved being. I would prefer him (dying) at a place where he is very happy," Nargeolet said, struggling to hold back her tears.