Oceans are the last unknown frontier on our planet. In a bid to explore the wreckage of Titanic, a tragic accident claimed the lives of five people aboard the Titan, a deep-sea submersible. The vessel imploded under the intense pressure of the Atlantic Ocean, near the wreckage of the Titanic, killing all passengers onboard instantly. Among the casualties were some of the most prominent figures in ocean exploration and business, including OceanGate CEO and founder Stockton Rush, and Pakistani millionaire Shahzada Dawood and his son.

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The Titan submersible, ostensibly designed to withstand the crushing pressures of the ocean depths, met a catastrophic end on June 18, turning a mission of exploration into a tragic disaster. The US Coast Guard confirmed the fatalities after a week-long search for survivors that drew attention worldwide.

The sub's mission was a dangerous one, to descend to the depths where the iconic Titanic shipwreck rests, more than 12,000 feet below the ocean surface. But, while the risks of deep-sea exploration are known, what exactly went wrong during the Titan's fatal descent remains a mystery. All that is known at the moment is that the submersible underwent a catastrophic implosion, as the ocean around it crushed the vehicle like an empty aluminium can within milliseconds.

Deep-sea vessels are built to resist extreme pressures, low temperatures, and the pitch darkness of the ocean's depths. But, they are not infallible. In the deep sea, the water pressure can reach more than 1,000 times that at sea level, enough to crush even the most robust vessels if not adequately prepared.

When we look out across the vast, blue ocean, we are only seeing the surface of an incredibly deep and complex ecosystem. The true depth of the oceans is a mystery to many, and beneath the tranquil surface, the conditions become far more dangerous than most people imagine. One key element that dramatically changes as we descend is water pressure.

Knowing a few important concepts is a precursor to understanding the deadly role that pressure plays in dee-sea diving, and the severity of the tragic implosion end of the Titanic sub Titan:

Ocean depth and water pressure

The ocean is divided into several layers, each with its own unique conditions. The layers are the sunlight zone, twilight zone, midnight zone, abyssal zone and hadal zone (the last being the deepest).

The sunlight zone is where you will find most marine life and human activity, ranging from the surface to around 200 metres deep. As we descend into the twilight zone (200 to 1,000 metres), sunlight diminishes, temperature drops, and pressure increases. In the midnight zone (1,000 to 4,000 metres), complete darkness reigns, and bizarre, adapted life forms thrive.

Further still, the abyssal zone (4,000 to 6,000 metres) covers the vast ocean floors. Here, temperatures hover just above freezing, and pressure is enormous. The deepest part, the hadal zone (6,000 to 11,000 metres), includes trenches and canyons. The Mariana Trench, the deepest part of the ocean, plunges down to 11,034 metres, more than Mount Everest's height.

Water pressure refers to the force exerted by the weight of water above a point. As you descend, every 10 metres increase in depth adds another atmosphere (atm) of pressure, which is equivalent to the pressure at sea level. At the deepest part of the ocean, it could reach up to 1,100 atmospheres of pressure, akin to having about 50 jumbo jets stacked on top of you.

What happened to the Titan?

This extreme pressure poses one of the biggest challenges to deep-sea exploration. When a submersible like the Titan descends, it has to withstand an increasingly heavy load of water pressing down on it. If the pressure outside the sub becomes significantly greater than the pressure inside, it can cause the vessel to implode.

A catastrophic implosion, like what was suffered by the Titan, is a sudden and devastating collapse. It is a violent event triggered by a severe pressure imbalance and can be likened to crushing an empty soda can. If you were to take an empty soda can, seal it, and then apply a massive force to it (like a car running over it), the can would rapidly collapse or "implode" due to the pressure difference.

But, why didn't the Titanic implode when it sank, while the Titan did during its descent? The answer lies in the way these structures were subjected to pressure. The Titanic was filled with air compartments when it sank, but as it gradually descended, these compartments likely filled with water, equalizing the pressure inside and outside the ship. This gradual pressure change prevented a rapid implosion.

In contrast, the Titan, being a sealed vessel full of air, descended rapidly. If the pressure inside the vessel didn't increase at the same rate as the pressure outside, which is a big challenge given the speed of the descent, it would have set up a dangerous pressure differential. If this differential exceeded the structural limits of the submersible, it could have triggered the catastrophic implosion that appears to have occurred.

Extreme deep-sea conditions pose a danger

These extreme conditions pose significant risks to both humans and machines. For humans, diving even a few hundred metres without the proper equipment can result in fatal conditions like nitrogen narcosis and decompression sickness, also known as "the bends". Deep-sea pressures can cause submersibles to implode, making exploration and rescue operations challenging.

For the creatures of the deep, these conditions are normal. They've evolved unique adaptations to survive, like bioluminescent lights to communicate and hunt in the pitch-black darkness, or flexible bodies to withstand the crushing pressure.

For that reason, most underwater deep-sea exploration is performed by unmanned vehicles. Among the most widely used deep-sea vehicles are Remotely Operated Vehicles (ROVs), Autonomous Underwater Vehicles (AUVs), and Manned Submersibles, the category the ill-fated Titan falls into. These vehicles are built with materials such as titanium and syntactic foam, specifically designed to withstand intense pressure. They also carry advanced life-support systems to safeguard the occupants and high-definition cameras to document the deep-sea world.