London — There has been much discussion about how to find the Titan, the tourist submersible that disappeared Sunday while carrying five people down to the wreckage of the Titanic, and how difficult it will be to locate the small vessel in the vast North Atlantic. But even if the monumental effort by U.S. and Canadian authorities to find the sub is successful, that's only the first challenge.

Rescuing the five people from the sub would be another challenge entirely. 

Without any sightings at the surface, the only clue rescuers had to go on by Wednesday afternoon was the sound described as banging noises detected by surveillance planes. That left open the possibility of a scenario where the submersible could still be found floating on or near the surface, which would allow rescuers to open it — it can only be opened from the outside — and rescue the crew.

But if it is on the sea floor near the Titanic wreckage, it will be a much more challenging proposition.

"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 affect a sub-to-sub rescue," Titanic expert Tim Matlin told the Reuters news agency.

Two possible rescue options

Ralf Bachmayer, a professor of marine environmental technology and deep-sea engineering at the University of Bremen in Germany, told CBS News on Wednesday there were two main possibilities for a rescue if the sub is on the sea floor. It could be winched up, which he said would be "very difficult" at the depth in the area, which is around 13,000 feet, or almost two and a half miles. 

Matlin cast doubt on the feasibility of such an operation, telling Reuters that rescuers "can't have a line tethering it all the way down because it would be too heavy and too much drag," given the length of cabling that would be required.

The other possibility could be a flotation device, which Bachmayer called a lift bag, that could be slid under the sub to help lift it to the surface.

But the Titan could be entangled in debris in the wide field of the Titanic's wreckage, and that type of rescue attempt would require remotely operated vehicles to gain sufficient access to slide such a device under the 21-foot-long submersible.

The use of ROVs, or Remotely Operated Vehicles, would be vital to either rescue option, should one be deemed viable.

In the first scenario, an ROV or multiple ROVs could be used to attach a cable to haul the Titan to the surface. They could also be used to dislodge it from anything it's caught on.

Blair Thornton, a professor of marine autonomy at the University of Southampton in England, said modern ROVs "typically have high-definition cameras, lights, and will be physically connected with a tether to a ship, with very experienced pilots operating them."

A rescue operation at the depth of the area around the Titanic shipwreck, he said, would likely involve attaching a cable to the stricken vessel with an ROV, and then winching it slowly to the surface.

But Thornton said this particular scenario would be a highly unusual task for available ROVs, and it would likely require them to be adapted to the challenges of operating so deep.

ROV operators, he said, are accustomed to "thinking on their feet, assessing the situation and trying to dislodge or move objects that were never designed to be grabbed by an ROV."

Bachmayer agreed that the missing Titan had presented rescuers with an unprecedented situation.

"We had the situation with the Kursk," said Bachmayer, referring to the nuclear-powered Russian submarine that sank in the Barents Sea in 2000. "They tried to get the people out. But that's not possible in this case, because it's much too deep, and the rescue submarine used then, it is not suitable for such depths."

None of the 118 crew members of the Kursk survived that disaster.

Both of the experts who spoke to CBS News said using ROVs at such depths had been done before — in order to retrieve instruments from the seabed, for instance — but that recovering a manned submersible vehicle would present new challenges, and even if and when the Titan is located, there will still be many questions to answer.

For example, "Is the vehicle's housing still intact?" asked Thornton. "What is keeping it on the bottom? The response will depend on those situations."

If the main housing, the body of the sub that keeps water out and oxygen in, is still intact, he said the sub would be essentially weightless in the sea, and as long as it isn't entangled, it should be moveable. On land, it would weigh several tons.

Thornton was quick to stress that finding the Titan was the essential first step, and he said it was undeniable that time was running out quickly, given the limited oxygen supply it went down with.

The U.S. Coast Guard said Tuesday that the submersible was believed to have enough air left to last the crew about 41 hours, and  rescue officials said they continued to hold out hope. But experts on the deep sea and on the Titanic didn't voice much optimism.

"Titanic's wreck is two and a half miles down. It's pitch black down there. It's freezing cold. The seabed is mud and it's undulating. You can't see your hand in front of your face. The only way you can find where you are is by a thing called sonar. Not even radar works," Matlin, who has authored three books on the Titanic disaster, told Reuters. "It's really a bit like a moon shot. ... So, I do fear for the lives of those explorers who are on board."

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• Rescue
• RMS Titanic
• Submarine