It has now been three days since contact was lost with a submersible diving towards the wreck of the RMS Titanic 3,800 meters below the surface of the sea.
The Titan, built and operated by an undersea adventure tourism company called OceanGate Expeditions, remained missing as of Wednesday evening. There are five people onboard: OceanGate CEO Stockton Rush, British billionaire Hamish Harding, renowned French river Paul-Henri Nargeolet, British-Pakistani businessman Shahzada Dawood, and his son Suleman.
The vessel set off for its excursion to the Titanic’s underwater grave 13,000 feet below the water’s surface on Sunday morning. It lost contact with its mother ship, the Polar Prince, after one hour and 45 minutes.
Follow the latest updates on the missing Titanic submarine here
A massive multinational search is now underway as officials say the five passengers will run out of oxygen by 5.30am ET (10.30am GMT) on Thursday.
More than 10,000 square miles of the North Atlantic have been searched so far, according to the US Coast Guard, with reports that regular “tapping sounds” were picked up by sonar both on Tuesday and Wednesday, providing fresh hope that the occupants are alive and Titan might yet be found.
Five additional rescue vessels will arrive in the next 24-48 hours, after the estimated time when the Titan’s oxygen reserves will have been exhausted. However, the French ship Atalante carrying the Victor 6,000 underwater (ROV) and winch — the only one capable of reaching the Titanic wreck 4,000m under the ocean surface — will have a narrow window of time to conduct rescue operations as it is only expected to reach the search site on Wednesday night.
Although we still don't know what might have happened to them, experts have cohered around three basic scenarios, none of which are good news for the crew – or their families.
Catastrophic implosion
Arguably the worse case scenario is a catastrophic rupture in the submersible's outer shell, causing an implosion that would kill everyone aboard very quickly.
The wreck of theTitanic sits around 3,800m below sea level, where the water pressure bearing down on any given object is roughly 376 times greater than that exerted by Earth's atmosphere.
Humans cannot survive that kind of pressure, which is why we need a submersible. But, according to Australian robotics professor Stefan B Williams, that level of danger makes the margin of error small.
"Although the Titan’s composite hull is built to withstand intense deep-sea pressures, any defect in its shape or build could compromise its integrity – in which case there’s a risk of implosion," wrote Mr Williams in The Conversation.
Industry insiders told The Guardian that the Titan would probably have reached a depth of around 3,500 meters by the time communication was lost, meaning the pressure would be equal to 345 times Earth's atmosphere.
The vessel had a real-time monitoring system to detect any problems with the hull. In theory, that would give its operators enough time to send word of a problem or begin procedures to make an emergency surfacing.
However, the Titan lost contact with its mother ship without any other indications of a problem, suggesting whatever happened may have happened very quickly.
“If something’s gone wrong, there’s a good chance it’s gone very wrong,” Mr Williams told The Guardian. “If the pressure vessel has failed catastrophically, it’s like a small bomb going off. The potential is that all the safety devices might be destroyed in the process.”
A similarly deadly scenario would be if there was a fire on board the Titan, perhaps started by a short circuit in one of its electrical systems.
If the Titan did not implode, it may still be trapped underneath the water with the occupants running out of oxygen, experts said.
Trapped underwater
There are many reasons for that, ranging from a somehow non-catastrophic leak to getting stuck on a piece of the Titanic wreckage, which is strewn about a wide area of the ocean floor.
Butch Hendrick, president and founder of the dive training company Lifeguard Systems, told CBS News that the submersible "could have gotten itself entangled somewhere", causing the communications antenna to become "dislodged" and "broken".
Frank Owen, a retired Royal Australian Navy official who specialises in submarine escape and rescue, likewise told The Guardian: "There are parts of [the Titanic] all over the place. It’s dangerous."
Although the Titan does not require any engine power in order to surface, getting caught on debris or taking on water could prevent it from doing so.
In that case, the crew would be forced to sit and wait for rescue in a region of the ocean too deep for any human diver or almost any other vehicle to reach.
"There's no opening a gate, there's no opening a window, there's no making an interactive lock," Mr Hendrick said.
As well as dwindling oxygen, the crew might be dealing with hunger and low temperatures. If the life support systems have failed, the interior of the Titan will be extremely cold, since no sunlight reaches that far down in the ocean.
And according to science journalist David Pogue, who travelled on the Titan last year, crew usually only have a sandwich and a bottle of water along with them.
Even if a rescue vehicle could get down there in time, it would be tough to spot the Titan. “The actual nature of the seabed is very undulating. Titanic herself lies in a trench. There’s lots of debris around," retired UK Royal Navy rear admiral Chris Parry told Sky News.
"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.”
Surfaced but lost
The best case scenario is that the Titan has actually surfaced, and is now floating on the Atlantic Ocean waiting to be spotted and rescued.
The Titan depends on constant communication with its mother ship in order to navigate, as it has no GPS or other guidance systems. During David Pogue’s trip on the mothership last year, though not while he was on the sub itself, that communication was lost for about five hours.
Yet according to CNN, the Titan uses ballast to stay underwater, and that ballast can be dislodged by occupants deliberately rocking the ship or by using a pneumatic pump to knock it free. The lines securing the ballast are also designed to fall apart after 24 hours.
Hence, if the only problem was that the sub had lost communication, or if something else had gone wrong that did not sink the sub, the crew should have been able to surface by now.
That would not mean they are out of the woods. According to reports, the Titan's hatch is bolted from the outside, and cannot be opened from the inside. Since the vessel is airtight, that would leave the crew still dependent on its oxygen supply.
Although the vessel reportedly has an array of equipment to help it be spotted on the surface, such as lights and reflectors, rescuers would still have a tough job locating it.
And on Monday, Mr Pogue said the sub did not, as far as he knew, have any kind of transmission beacon similar to those used on other boats to help rescuers find their way.
