Two-and-a-half miles down. Three-thousand, eight hundred and twenty-one metres. The pressure outside is three-and-a-half tons per square inch. These windows are nine inches thick, and if they go, it’s sayonara in two micro-seconds.”

That’s a quote uttered by fictional undersea treasure hunter Brock Lovett in James Cameron’s Titanic as he narrates a submersible exploring the sides of the 1912 wreck. The clip is circulating online due to its eerie relevance to the tragic end to the search for the Titan submersible, which went missing on Sunday AEST with five passengers aboard as they travelled to the depths of the North Atlantic.

After an undersea robot discovered debris from the submersible about 487 metres from the Titanic’s bow, at the wreck’s depth of 3.8 kilometres, the US coast guard concluded the vessel suffered a “catastrophic implosion”, killing the five men and ending any hopes of a rescue.

Officials have said it’s too early to tell when the implosion occurred and how. But what conditions on the ocean floor could’ve caused such a catastrophic end?

Why is the pressure so extreme in the deep sea?

All of us are exposed to atmospheric pressure, caused by the weight of the air above us pressing down. You can imagine it as a long column of air above you reaching to the top of the atmosphere, pressing down on the top of your head, according to the American Museum of Natural History.

The higher you go, the less of that column is pressing down on your body, which is why there’s lower pressure at higher altitudes.

At sea level, the pressure is at one atmosphere, which is equivalent to about one kilogram of weight pressing down on every square centimetre, or 14.7 pounds per square inch (PSI).

In the sea, that column of atmosphere pressing down is increased by the weight of water above you too – and water is far heavier than air. For every 10 metres of ocean depth, the pressure ramps up by one atmosphere.

At the depth of the Titanic wreck, the Titan submersible would have contended with pressure of between 375 and 400 atmospheres. The pressure is equivalent to about 2500 kilograms (5500 pounds) of force pressing in on every square inch.

Put another way, it’s the equivalent of more than 4000 tonnes applied on an area of one square metre, says Associate Professor Eric Fusil, director of the Shipbuilding Hub at the University of Adelaide.

The effect of this pressure was illustrated by Australian marine archaeologist Emily Jateff, who strung a bag of styrofoam cups to the outside of the submersible she travelled to the Titanic within in 2005. The cups were crushed to an eighth of their size.

At such depths, says Fusil, “it’s like crushing a can of soda to the size of a very small marble in your hand.”

What could have caused the implosion?

A catastrophic implosion of the kind that destroyed the Titan is “a phenomenon where, basically, you are crushing a pressure vessel from the outside. It results in the instantaneous implosion of that vessel, killing everyone within 20 milliseconds,” Fusil says.

“It is instant. The human brain is not able to process information at that speed. They wouldn’t have realised what happened.”

The massive difference in pressure between the inside of the hull, at one atmosphere, and 400 atmospheres in the depths outside the vessel means one tiny fracture or imperfection could have led the Titan to implode.

“This is why you need shapes like spheres, cylinders, that are naturally prone to perfectly balance the stresses inside the material – and then you need the materials to be able to sustain those high stresses,” Fusil says.

Other submersibles that travel to extreme depths are built with a single metallic material such as titanium. “Titanium is used because that’s material with a high-yield strength – it can absorb deformation very easily under a huge range of pressures. The titanium pressure vessel would just contract … and then when you go back up it would come back to its original shape without any permanent deformation. That’s called plastification.”

But the Titan was made from two materials: titanium end caps and a composite carbon fibre body.

“[Carbon fibre] is very stiff,” says Fusil. “It does not want to move. So Titan has a combination of two materials with opposite behaviour.

“If you’ve got those two opposite materials, maybe – and this is speculation – there was a defect created in the bonding of the composite material and the titanium part.”

Fusil’s analysis was backed up by retired US navy captain Alfred McLaren. “When you take a couple of dissimilar materials like that, and try and seal them, and then you change depth and temperatures and all that, the molecules are going to react differently,” McLaren told ABC radio.

He added the risk of a defect increases with every deep sea journey by a submersible. “All you had to have is just a hole [in the submersible’s seal] not much bigger than the diameter of your hair ... just a slice.

The passengers would’ve been killed instantly by the folding metal in the implosion or by the extreme pressure crushing the air-filled cavities of their bodies, including their lungs. “That thing would flood so quickly,” McLaren said. “They’d be gone in a fraction of a second.”

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