Victor 6000: The robot that is only hope to rescue missing sub from sea floor

Only one vehicle engaged in the search for the submersible that went missing on a voyage to the shipwreck of the Titanic has the ability to reach the bottom of the ocean, a marine expert has said.

British Antarctic Survey marine geophysicist Dr Rob Larter said: "It is incredibly difficult to find an object of this sort of size at the bottom of the ocean."

He added that the Victor 6000 remotely operated vehicle (ROV) is, he believes, the "only thing" that is on-site with the capability to get down to the sea bed and explore.

It is understood the Victor 6000 is now on the ocean floor searching for the missing sub.

The Titan submersible, operated by OceanGate Expeditions, was reported missing on Sunday evening in the mid-Atlantic about 435 miles south of St John’s, Newfoundland, Canada.

The five people on board the submersible may have as little as 20 hours' worth of emergency oxygen left as Thursday morning, officials said.

Read more: Live updates after Titanic tourist submersible goes missing

They include British billionaire Hamish Harding, 58, and UK-based Pakistani businessman Shahzada Dawood, 48, and his son, Suleman, 19.

French explorer Paul-Henri Nargeolet, who works for OceanGate, is also thought to be on board, as is OceanGate founder and chief executive Stockton Rush.

The firm charges $250,000 (£195,000) for an eight-day trip that includes a dive to the wreck of the Titanic.

But those who pay for the trip must also sign a waiver that clearly illustrates the risks involved.

New York-based writer Mike Reiss, who has worked on TV shows such as The Simpsons, went on the Titan last year.

He told BBC Breakfast on Tuesday about the waiver he had to sign before getting on board the submersible.

"You sign a massive waiver that lists one way after another that you could die on the trip," he said.

"They mention death three times on page one and so it's never far from your mind. You try to put it out.

"As I was getting on to the sub my thought was this could be the end.

"So nobody who's in this situation was caught off guard. You all know what you're getting into.

"It is really exploration. It is not a vacation. It's not thrill-seeking, it's not sky diving. These are explorers and travellers who want to see something."

The Victor 6000 ROV has been deployed to the area from the French ship L’Atalante and is the only submersible capable of reaching the depths of the Titanic.

The rescue operation is also searching the surface to see if the Titan managed to ascend but lost the ability to make contact.

Prof Greig said the ROV will not be able to lift the Titan itself and will have to attach “some device” to it.

He speculated that if Titan is trapped under debris then the ROV’s “manipulator arms” may be able to free it for the submersible to then make its own way up.

In a report on the Titan submersible by CBS last November, widely shared on social media since it went missing, journalist David Pogue read out part of the waiver.

He said it stated: "This experimental vessel has not been approved or certified by any regulatory body, and could result in physical injury, emotional trauma, or death."

A number of news reports about the Titan in the past year show just how cramped it is on the inside.

The vessel has no seats and is operated by a "PlayStation" style controller.

It also emerged that the submersible is locked with bolts from the outside, so there is no way for those on board to escape without assistance.

Reiss said he made one dive to the Titanic shipwreck and two more off the coast of New York.

“I’m not optimistic just because I know the logistics of it," he said.

"And I know really again, how vast the ocean is, and how very tiny the craft is.

“So the idea is, if it’s down at the bottom, I don’t know how anyone’s going to be able to access it, much less bring it back up."

Read more: Inside cramped Titan submersible missing during voyage to Titanic shipwreck

The search and rescue operation, involving military aircraft 900 miles east of Cape Cod, continued on Tuesday.

The US Coast Guard said the Canadian research vessel Polar Prince and 106 Rescue wing will continue to conduct surface searches alongside two US C-130 flights.

Watch: Search continues for Titanic tourist submersible missing in Atlantic