A deep-sea explorer and the close friend of the pilot of a Titanic tourist submarine that has been missing for days has said he is ‘optimistic’ for a ‘miracle’.

A rescue mission has been ongoing since Sunday to try and find the OceanGate Expeditions submersible Titan, which was carrying five passengers when it went missing in the Atlantic Ocean, including British billionaire Hamish Harding and pilot and renowned French diver Paul-Henri Nargeolet.

With just hours of oxygen left in the submarine, hope remains as search teams said they heard ‘underwater noises’ giving them hope that there were potential signs of life.

Speaking on Thursday’s Good Morning Britain with Susanna Reid and Ben Shepherd, deep-sea explorer Dr David Gallo said that his hope had been ‘sliding down’ until the noises appeared.

‘Everything’s happening very quickly, so it’s a race against time.

‘My hopes are high, and we need a miracle at this point but miracles do happen, so I’m very optimistic, I have to be.’

Paul-Henri Nargeolet is one of the leading experts on the Titanic shipwreck (Picture: Fivedeeps/https://.com/home/team/partners/caladan/ph/)

Talking about his close friend, pilot of the submarine Nargeolet, he said: ‘He is, and I mean this literally, as comfortable on a deck of a ship in a roaring hurricane, with waves crashing over the deck, he’s as comfortable there as he is sitting in a freezing cafe.

‘He’s incredibly kind, giving, it’s not just me that has this attachment, hundreds of people adore this man.

‘Extremely competent and unflappable, so if he were in a difficult situation, I’ve been there with him in difficult situations.’

‘He’s a pretty remarkable guy,’ he added.

‘He’s a pretty remarkable guy,’ Dr Gallo praised (Picture: ITV)

Dr Gallo continued: ‘If I was going to be stuck in a submarine with someone, it would be P H Nargeolet for sure.’

The explorer elsewhere called the situation a ‘bitter’ one, saying, about the rescue mission: ‘The best person to have on the job would be P H Nargeolet and unfortunately he’s not on this side of the equation with me, he’s on that side of the equation, which makes this very, very, bitter, to have to go this way.’

Good Morning Britain airs weekdays from 6am on ITV1.

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‘He’s a pretty remarkable guy.’