The sub was built to withstand such pressure - and experts will now be trying to determine what exactly went wrong. Analysis of the debris may help to establish this.

Titan is believed to have been 3,500m below sea level when contact was lost.

The vessel was so deep that the amount of water on it would have been equivalent to the weight of the Eiffel Tower, tens of thousands of tonnes.

If there were a rupture to the structure, the pressure outside would be much greater than the one inside the hull, compressing the vessel.

When a submarine hull collapses, it moves inward at about 1,500mph (2,414km/h) - that's 2,200ft (671m) per second, says  an expert.

The time required for complete collapse is about one millisecond, or one thousandth of a second.

A human brain responds instinctually to a stimulus at about 25 milliseconds. Human rational response - from sensing to acting - is believed to be at best 150 milliseconds.

The air inside a sub has a fairly high concentration of hydrocarbon vapours.

When the hull collapses, the air auto-ignites and an explosion follows the initial rapid implosion.

Human bodies incinerate and are turned to ash and dust instantly.

Any investigation is sure to focus on the Carbon Fibre mid-section of the Titan sub.

The pressure vessels of deep vehicles like this are normally constructed from a robust metal such as titanium and are shaped in a sphere, to spread the immense pressure equally around the passenger compartment.

But to fit more people inside, the OceanGate sub adopted a cylindrical shape, with a carbon fibre tube inserted between to titanium end caps. Carbon fibre is very tough - they use it to build aeroplane wings and racing cars.

But did that immense pressure at depth - more than 300 times the atmosphere at the sea surface - play on the material to expose flaws in the original fabrication or to introduce and then worsen instabilities over repeated dives

Aircraft are subjected to regular, very fine-scale inspections to ensure their materials are not developing cracks or that their layers are not starting to separate.

Photographing the Titan debris found on the ocean floor and bringing them back to the surface for study in a forensic lab may allow engineers to identify where on the sub structural integrity was lost, initiating the catastrophic implosion.

Previous passengers recall ill-fated Titan: 'I 100% knew this was going to happen' 

In the wake of the Titan's fatal implosion near the Titanic shipwreck on Sunday, some people who embarked on the company's deep-sea expeditions described experiences that foreshadowed the tragedy and look back on their decision to dive as "a bit naive.” But others expressed confidence and said that they felt they were "in good hands" nearly 13,000 feet (3,962 meters) below the ocean's surface.'Like playing Russian roulette' “I 100% knew this was going to happen,” said Brian Weed, a camera operator for the Discovery Channel’s “Expedition Unknown” show, who has felt sick to his stomach since the sub's disappearance Sunday. 

Weed went on a Titan test dive in May 2021 in Washington state's Puget Sound as it prepared for its first expeditions to the sunken Titanic. Weed and his colleagues were preparing to join OceanGate Expeditions to film the famous shipwreck later that summer. They quickly encountered problems: The propulsion system stopped working. The computers failed to respond. Communications shut down. Rush, the OceanGate CEO, tried rebooting and troubleshooting the vessel on its touch screens.“You could tell that he was flustered and not really happy with the performance,” Weed said. “But he was trying to make light of it, trying to make excuses.” They were barely 100 feet (30 meters) deep in calm water, which begged the question: “How is this thing going to go to 12,500 feet — and do we want to be on board?" Weed said.