Five people were on board the vessel when it went missing on Sunday. An urgent international operation is under way to try and locate the craft before its 96-hour emergency supply of oxygen runs out.

Search and rescue experts are facing a number of challenges, including not knowing if the Titan submersible is still under water or has managed to make its way back to the surface.

Sky News spoke to former Royal Navy Rear Admiral Chris Parry about the difficulties in bringing the five people home safe.

Finding the Titan in the dark

"The basic problem right up front is there doesn't seem to be a transmitting distress beacon" coming from the Titan submersible, Mr Parry told Sky News.

This means that rescue technology that looks for signals coming from stricken vessels - similar to locating an aircraft's black box - will struggle to find it.

A search of the seabed where the Titanic lies - around 12,500ft below the surface - will require a specialised submersible craft like an ROV (remotely operated vehicle).

But even then, it won't be easy.

"It's utterly dark down there, and you have also got a lot of mud and other stuff getting swept up by the (vehicle's) propulsors," Mr Parry said.

"You will see about 20 feet in front of you with search lights."

"The Titanic is a big ship so if you start at one end you are going to have to come up again pretty quickly," he said, adding it would require multiple trips to and from the wreck to search the whole area.

In addition, there's a field of debris around the Titanic meaning the seabed is uneven and covered in obstacles.

Read more:
The people on board missing Titan submersible

'Like being in space - with same degree of danger'

Mr Parry, a former NATO and naval commander, said the Titan went missing in "harsh conditions".

"Down there you are not dealing with weather at all. What you are dealing with is very strong ocean currents which are pushing you along."

He said the submersible could be rolled along the seabed, such is the strength of the currents at that depth.

"It's rather like being in space, the same degree of danger and the same degree of issues.

"Outside the craft you have got about 5,000lbs (2,268kg) of pressure per square inch on the hull."

In the event the Titan submersible is located on the seabed or deep below the surface it will be difficult to bring those aboard back up safely, Mr Parry said.

He suggested a vehicle with a crane or grab type capability, or some other way of attaching to the hull, would be required.

It is believed that the submersible cannot be opened from the inside.

Could Titan be on the surface?

A better scenario would be finding the Titan craft intact and on the surface, Mr Parry said.

"If they managed to use their compressed air to blast themselves to the surface it's a question of someone being able to find them wherever they are.

"They should have found them by now," he added.

Alistair Greig, a professor of marine engineering at University College London, described the process by which this would be achieved.

He said submersibles typically have a drop weight - "a mass they can release in the case of an emergency to bring them up to the surface using buoyancy".