The North Atlantic Ocean, where the Titan submersible was launched on a fateful trip to view the wreck of the Titanic, is inhospitable, at times outright hostile.
And that’s before you even venture below the waves.
Take a 3,800-metre journey to understand why seeing the Titanic remains such a rare and dangerous adventure.
One of the first big challenges of a deep sea expedition is manoeuvring a submersible from the deck of a ship into the water. Usually, a huge ship equipped with a crane is the answer.
But in the case of the Titan, it was lowered into the water on a platform, and then scuba divers unclipped it once it was submerged. This allowed it to operate from a smaller ship.
At the Earth’s surface, you’re experiencing one atmosphere’s worth of pressure. But under the ocean, the pressure increases by the equivalent of one atmosphere for every 10 metres you go down.
So at a depth of just 200 metres, the pressure is already 20 times greater than it is at sea level. For comparison, the pressure inside a car tyre is only roughly double the pressure we feel on the surface.
This intense pressure is the most significant risk to deep-sea missions.
Ahmed Gabr undertook the deepest scuba dive on record in 2014. He reached 332 metres below the surface of the Red Sea.
The operations of military submarines are classified but experts say they typically max out at depths about 500 metres.
The deepest-ever submarine rescue occurred at 480 metres in 1973, when two men were saved from the submersible Pisces III on the seabed off Ireland.
Deep sea expeditions have to grapple with entanglement hazards — being tangled up in a fishing net or caught underneath a rock overhang could stop a mission from being able to return to the surface.
Submersibles rely on ballast to adjust their buoyancy and return to the surface; this involves dropping weight when it’s time to ascend. The Titan reportedly had seven levels of fallbacks, so it could surface even if some of its systems failed.
At a depth of about 1,000 metres, things change. The ocean’s ‘midnight zone’ begins, as sunlight no longer penetrates from the surface.
At these depths, the temperature also drops to 4 degrees Celsius. If a submersible suffers power loss, its passengers would face extreme cold.
Sydney engineer Ron Allum designed the submersible that took Titanic filmmaker James Cameron to the bottom of the Mariana Trench, the deepest part of the ocean.
“When you get beyond 1,000m,” he explains, “you’re getting to the point where if something goes wrong it’s going to be quite catastrophic.
“As an adventurer … I guess you’re trying to be self-sufficient. You don’t rely on others for rescues. So you’re trying to justify within yourself that you’ve considered all the factors.”
Experts had previously warned the company operating the Titan submersible, OceanGate, that without industry oversight it was exposing passengers to possible catastrophic failure. And a former employee raised concerns that parts of the vessel were only rated for use down to 1,300 metres.
An American journalist who joined a Titan expedition last year reported signing a disclaimer that stated: “The experimental submersible vehicle has not been approved or certified by any regulatory body. Any failure could cause severe injury or death.”
In an enclosed space, there are two big challenges around storing oxygen for passengers to breathe and capturing the carbon dioxide they breathe out.
The Titan was equipped with enough air for 96 hours, and a scrubber to remove CO2. If CO2 levels get too high aboard a submersible, passengers will lose consciousness.
Any vehicle traversing these depths will have to contend with unpredictable and variable currents. In the area where the Titanic lies, the North Atlantic Deep Water current sweeps through waters from about 1,500 metres to 4,000 metres.
Fire is one of the biggest dangers undersea vehicles can face. If an electrical fault causes even a small fire it can create noxious fumes and burn through oxygen supplies.
Because radio waves struggle to penetrate salt water, communication underwater relies on SONAR (measuring vibrations), and so is far more limited. The Titan could only send basic text messages.
Ron Allum says to withstand the pressure at these depths, any structure must be perfect.
“As an engineer you’re looking at geometric shapes. You might have a cylinder or a sphere, which are the best shapes for withstanding pressure, but if that cylinder becomes out-of-round, there’s a little dent in it, all of a sudden the thing is going to collapse like a pancake and become flattened.
“Remember all the pressure on the outside is trying to get inside into this little pressure sphere which may be the capsule where people are going to be.
“On something like the Titan submersible, it’s a very large end cap. At Titanic depths, there’s over 9,000 tonnes of pressure just on the end cap, probably 50,000 to 60,000 tonne on the hull. So we’re talking immense forces … It’s just mind-boggling.”
This environment is intensely hostile to humans, but other species thrive. The cuvier’s beaked whale holds the record for the deepest recorded dive by a mammal at an astonishing 2,992 metres.
A wild collection of fish, eels, sponges, worms, jellyfish and even sharks also live at these depths.
The debris of the Titan has been discovered on the ocean floor several hundred metres from the Titanic wreck, with all onboard believed to be dead.
In the event that it had been found intact deep underwater, then a remote-operated sub like the French Victor 6000 on a long cable would have been needed to attach a line to it and winch it up to the surface.
ROVs can operate to depths much deeper than the Titanic wreck and have been used to successfully raise fighter jets and smaller ships.
Most scientific and industrial work done at these depths involves submersibles that don’t carry humans.
Professor Stefan Williams from the Australian Centre for Field Robotics says you can often see as much using cameras on robotic vehicles that are operated remotely.
“A manned submersible, they have their niche. I think it gives people that sense of being there.
“But you’re looking through a tiny little porthole, you don’t have a broad expanded view, light attenuates pretty quickly, so you kind of can see what’s right in front of you.”
Experts estimate the Titan submersible lost contact with the surface when it was about 3,500 metres down. Importantly, some of the Titan’s navigation systems were reliant on communications with the surface to operate.
The Titanic hit the ocean floor around 2:30 in the morning on April 15 in 1912.
More than a century on, the world has changed beyond recognition, but these depths remain at the very outer limits of the human experience.