An expert has given three possible explanations as to what happened to the submarine-like vessel that vanished in the middle of the Atlantic Ocean.
Four people and the vessel's pilot paid around $250,000 (around £195,000) in the hope of seeing the Titanic shipwreck which lies 12,500ft beneath the Atlantic's surface, off the coast of Canada.
But the OceanGate Expeditions vessel lost contact around an hour and 45 minutes after it submerged on Sunday and rescuers warn they may not reach the ocean craft before its oxygen supply runs out.
Speaking to SkyNews, former Rear Admiral [senior naval flag] Chris Parry said: "It's very concerning indeed. Having listened to the timescale it's very worrying."
He outlined three possible explanations for the submersible losing contact and explained the "only hope" they had in rescuing those in the underwater craft.
Umbilical connection lost
Parry said the "optimistic" option is that they "lost an umbilical connection with the surface."
The vessel is called Titan and notably is a submersible, not a submarine. Unlike submarines that leave and return to port under their own power, submersibles require a ship to launch and recover them.
Titan's location is unknown because it lost contact with its support ship, Polar Prince, as CNN reported.
So the umbilical cords generally connect to the surface facility.
Communication malfunction
Contact with the Titan submersible was lost 1 hour 45 minutes into its dive on Sunday afternoon and Parry wonders if there has been a "malfunction" and the submarine is actually still continuing to operate but "obviously out of contact with its mother ship."
The vessel uses Elon Musk’s Starlink satellite technology to communicate, though it is unclear if it was the cause of the loss of contact.
OceanGate tweeted last week: "Without any cell towers in the middle of the ocean, we are relying on @Starlink to provide the communications we require throughout this year’s 2023 Titanic expedition."
If there was a power or communications failure, the Guardian reports that the Titan would be equipped with drop weights, which can be released in an emergency, creating enough buoyancy to take it to the surface.
Catastrophic failure
The worst option, "at the other end of the scale", Parry said is that there "could have been an accident."
He continued: "It could have become entangled in the wreckage of the Titanic. It could indeed have had a catastrophic failure."
He said the seabed is very undulating and the Titanic ship lies in a trench, so there’s lots of debris around.
He continued: "So trying to differentiate with sonar in particular and trying to target the area you want to search in with another submersible is going to be very difficult indeed."
An OceanGate Expeditions statement read: "We are exploring and mobilising all options to bring the crew back safely. Our entire focus is on the crewmembers in the submersible and their families.
"We are deeply thankful for the extensive assistance we have received from several government agencies and deep sea companies in our efforts to reestablish contact with the submersible.
"We are working toward the safe return of the crewmembers."
Journalist Steve Norris added that attempts to contact the submarine had proven unsuccessful.
British Prime Minister Rishi Sunak said his thoughts are with the families of those involved.
The Prime Minister's official spokesman said: "The families involved will be deeply concerned and his thoughts are with them and the Foreign Office is providing support."
He said that the Prime Minister "clearly wants to pass on his thanks to those that are responding" to the situation.
