The search for the missing Titanic sub and how it might be found

Rescue teams are in a race against time after a tourist submersible carrying five people went missing during a dive to the wreck of the Titanic.

But how are rescuers trying to find a vessel that has already been missing for nearly three days in the vast depths of the Atlantic Ocean and what will the challenges be when they do?

The crew of the Titan submersible lost contact with its surface vessel - the Polar Prince - an hour and 45 minutes after it began a dive to see the wreck on Sunday. The sub's air supply is expected to run out at about 11:00 BST (10:00 GMT) on Thursday.

The Titanic wreck lies some 435 miles (700km) south of St John's, Newfoundland, and about a dozen ships are at the scene or on the way to help the rescue mission but changeable weather and poor visibility are hampering efforts.

Prof Alistair Greig, a submarine expert from University College London, says one of the big problems is that rescuers do not know whether to look on the surface or the seabed - it is "very unlikely" to be in between - and warns searching each poses challenges.

If the sub can't send distress signals for any reason, Prof Greig says: "It's about the size of a large transit van and it's painted white so trying to see that from the air... is going to be a real challenge."

US and Canadian agencies, navies and commercial deep-sea firms are all helping the rescue operation, which is being run from Boston, Massachusetts, using military planes and sonar buoys to search an area of more than 20,000 sq km.

The Polar Prince has already been joined by the cable-laying ship Deep Energy and three other ships, while more are on the way.

Some carry unmanned remotely operated vehicles (ROVs) that will be needed to scour the sea floor.

The Horizon Arctic supply ship is taking potentially crucial equipment to the area, including a US Navy Flyaway Deep Ocean Salvage System - a special winch system designed to provide reliable deep ocean lifting capacity for the recovery of large and heavy undersea objects.

Vessels involved in Titan search operation

Polar Prince (on site) - Research vessel that launched the Titan submersible

Deep Energy (on site) - Commercial cable-laying ship with two ROVs that can operate down to 3,000m (10,000ft)

Atlantic Merlin (on site) - Canadian offshore supply vessel with 4,000m winch system. Carries ROVs but it's unknown how deep they can operate

Skandi Vinland (on site) - Subsea support vessel with two ROVs but it's unknown how deep they can operate

L'Atalante (due to arrive late on Wednesday) - French-operated ship heading to the scene with an ROV that can operate at depth of Titanic wreckage

Horizon Arctic (en route) - Commercial vessel loaded with winch system

Glace Bay (in vicinity on standby) - Canadian naval ship carrying decompression chamber and ready to provide medical assistance

John Cabot (on site) - Scientific research vessel with sonar search capabilities operated by the Canadian Coast Guard

Ann Harvey (en route) - Light icebreaker vessel operated by the Canadian Coast Guard

Terry Fox (at St John's) - Heavy icebreaker vessel operated by the Canadian Coast Guard

Rescuers must also explore depths that could reach nearly 4km (2.5 miles) for the 6.7m (22ft) long submersible - because radio and GPS signals can't travel through water.

Canadian P-3 Aurora aircraft have been conducting sonar searches and sonar buoys are also being deployed in the area.

Sonar buoys, or sonobuoys, detect and identify objects moving in the water - and are often used in the hunt for enemy submarines.

They either listen for sounds produced by propellers and machinery (passive detection) - which could also include the crew making noise against the hull of the sub - or by bouncing a sonar "ping" off the surface of the vessel (active detection) and listening for the returning echo.

The more buoys that pick up the signal the more accurately they can work out the location of the source - by timing how long the signal takes to reach each buoy and then working out how far away each is.

The US Coast Guard said a Canadian aircraft had detected "underwater noises" several times on Tuesday night and on Wednesday morning but could not confirm US media reports saying it had detected underwater "banging sounds" in 30-minute intervals.

Any attempt to search the ocean floor in that area would probably be carried out by an unmanned remotely operated vehicle (ROV).

Two ROVs are actively searching for the Titan, focusing on the area where noises were detected.

Those ROVs can reportedly reach depths of up to 4,000m - enough to reach the Titanic which lies at 3,800m (12,500ft) beneath the surface - and will soon be joined by an additional ROV from French research vessel L'Atalante described by Capt Jamie Frederick of the USCG as "state of the art".

If the Titan is on the seabed and can't get back up under its own power, the options are very limited, according to Prof Greig.

"If it is deeper than more than 200m (656ft) there are very few vessels that can get that deep, and certainly not divers. The vehicles designed for navy submarine rescue certainly can't get down to anywhere near the depth of the Titanic."

But ocean recovery expert David Mearns says that if an ROV can locate the Titan then it should be able to recover it. "A world-class ROV with twin manipulators can actually grab hold of [the Titan] or attach a lift line to it and slowly haul it to the surface," he adds.

The US has previously used ROVs and the Flyaway Deep Ocean Salvage System in salvage operations at the depth of the Titanic, including to locate and recover a crashed fighter jet from a depth of 3,780m (12,400 feet) in the South China Sea last year.

In that case, the US military used the ROV to attach rigging around the aircraft and connect it to a lifting hook that was lowered from a crane on the rescue vessel on the surface.