The search for a tourist submersible carrying five people has gone deep, with a remotely operated vehicle diving to the last known location before contact was lost.

The ROV carries a camera and will scour the depths of the Atlantic Ocean where the vessel remains missing for a third day after setting off from Newfoundland to explore the Titanic wreck.

France and Britain have joined the search-and-rescue effort, as the identities of all five people on the Titan craft were revealed by media.

They include Stockton Rush, the founder and CEO of OceanGate Expeditions, the company behind the tourism venture.

The other four on the expedition – which costs $US250,000 ($367,000) per person – are British billionaire Hamish Harding, Pakistani-British businessman Shahzada Dawood with his son Suleman and French explorer and Titanic expert Paul-Henri Nargeolet, 77.

Mr Dawood senior is a close associate of the King, having worked extensively with the Prince’s Trust. British media is reporting the King has requested regular updates on the Atlantic search.

In a grave update on Wednesday morning (AEST), the US Coast Guard said the lost vessel would have about 40 hours of oxygen left – less than half its initial supply.

Commander Harry B. Harris Jr. said the fact that it hadn’t surfaced on its own was “indicative of a problem”.

He said the search would be complicated by the fact that the experimental vessel was not emitting any sound.

“Because it’s probably not making noise… to find it acoustically will be a challenge,” he said.

The US Coast Guard said it had searched an ocean surface area of 7600 square miles (19,600 square kilometres) – bigger than the US state of Connecticut.

“These search efforts have not yielded any results,” said Captain Jamie Frederick.

“We are out there, we are searching.

“If the sub is located, the experts will look at the best course of action for recovering the sub.”

Obstacles and complications

Rescuers face significant obstacles both in finding the Titan and in saving the people aboard, according to experts.

If the submersible experienced an emergency in mid-dive, the pilot would likely have released weights to float back to the surface, according to Alistair Greig, a professor of marine engineering at University College London.

But without any communication, locating a van-sized submersible in the vast Atlantic Ocean could prove challenging, he said.

The submersible is sealed with bolts from the outside, which means the occupants cannot escape without assistance even if it surfaces.

If the Titan is on the ocean floor, a rescue effort would be even more challenging due to the extreme conditions below the surface.

The Titanic lies 3810 metres underwater, where light does not penetrate.

Only specialised equipment can reach those depths without getting crushed by the massive water pressure.

“It’s really a bit like being an astronaut going into space,” said Tim Matlin, a Titanic expert.

“I think if it’s on the seabed, there are so few submarines that are capable of going that deep. And so, therefore, I think it was going to be almost impossible to effect a sub-to-sub rescue.”

US Coast Guard Captain Jamie Frederick says the search has not had any results yet. Photo: Getty

OceanGate said it was “mobilising all options” to rescue those aboard the Titan.

US coast guard Rear Admiral John Mauger told NBC News the company is leading the search efforts with coast guard assets brought to the site.

“They know that site better than anybody else,” Mauger said.

“We’re working very closely with them to prioritise our underwater search efforts and get equipment there.”

OceanGate schedules five week-long “missions” to the Titanic each summer, according to its website.

David Pogue, a CBS reporter, dove to the site on board the Titan last year.

In a December news report, he read aloud the waiver he had to sign, which noted the submersible had “not been approved or certified by any regulatory body” and could result in death.

In an interview on Tuesday, Pogue said the OceanGate has successfully gone down to the wreck about two dozen times and that the company does a meticulous safety check before each attempt.

“They treat this thing like a space launch,” he said.

“It is definitely a culture of safety.”

The wreckage of the Titanic, which sank in 1912 after hitting an iceberg, lies at about 3800 metres.

The British passenger ship sank on its maiden voyage, killing more than 1500 people, a tragedy that has been immortalised in books and films, including the 1997 blockbuster movie Titanic.