The disappearance of the submersible Titan during a visit to the wreck of the Titanic has raised questions about just what risks are involved in such an expedition to the deep.

At some point in Autumn 1911, an enormous chunk of ice cleaved away from a glacier on the southwest of Greenland's vast ice sheet. Over the following months, it slowly drifted south, melting gradually as it was carried by the ocean currents and the wind.

Then, on the cold, moonless night on 14 April 1912, a 125m-long (410ft) iceberg – all that remained of the estimated 500m (1,640ft) chunk of ice that left a fjord in Greenland the previous year – collided with the passenger ship RMS Titanic as it made its maiden voyage from Southampton in the UK to New York, USA. In under three hours the ship had sunk, taking more than 1,500 passengers and crew to their deaths. The wreck now lies nearly 3.8km (12,500ft) beneath the waves at a site nearly 400 miles (640km) southeast of the Newfoundland coast.

Icebergs still pose a hazard to shipping – in 2019 1,515 icebergs drifted far enough south to enter transatlantic shipping lanes during the months of March to August. But the Titanic's final resting place carries dangers of its own, meaning visits to the world's most famous shipwreck present a significant challenge.

With the disappearance of a five-person submersible while carrying paying passengers on a trip to the Titanic wreck, the BBC looks at what this region of the ocean floor is like.

Navigating in the deep

The deep ocean is dark. Sunlight is very quickly absorbed by water and is unable to penetrate much deeper than about 1,000m (3,300ft) from the surface. Beyond this point, the ocean is in perpetual darkness. The Titanic lies within a region known as the "midnight zone" for this very reason.

Previous expeditions to the wreck site have described descending for more than two hours through total darkness before the ocean floor suddenly appears beneath the lights of the submersible.

With limited line of sight beyond the few metres illuminated by the truck-sized submersible's onboard lights, navigating at this depth is a challenging task, making it easy to become disoriented on the seabed.

Detailed maps of the Titanic wreck site put together by decades of high-resolution scanning, however, can provide waypoints as objects come into view. Sonar also allows the crew to detect features and objects beyond the small pool of light illuminated by the submersible.

Submersible pilots also rely upon a technique known as inertial navigation, using a system of accelerometers and gyroscopes to track their position and orientation in relation to a known starting point and velocity. OceanGate's Titan submersible carries a state-of-the-art self-contained inertial navigation system which it combines with an acoustic sensor known as a Doppler Velocity Log to estimate the depth and speed of the vehicle relative to the sea floor.

Even so, passengers onboard previous trips to the Titanic with OceanGate have described just how hard it is to find their way upon reaching the ocean floor. Mike Reiss, a TV comedy writer who worked on The Simpsons and took part in a trip with OceanGate to the Titanic last year, told the BBC: "When you touch bottom, you don't really know where you are. We had to flail around blindly at the bottom of the ocean knowing the Titanic is somewhere there, but it is so pitch dark that the biggest thing under the ocean was just 500 yards (1,500ft) away and we spent 90 minutes looking for it."

Crushing depths

The deeper an object travels in the ocean, the greater the pressure of the water around it grows. On the seabed 3,800m (12,500ft) underwater, the Titanic and everything around endures pressures of around 40MPa, which are 390 times greater than those on the surface.

"To put that into perspective, that is about 200 times the pressure of what is in a car tyre," Robert Blasiak, an ocean researcher at the Stockholm Resilience Centre at Stockholm University, told the BBC Radio 4's Today programme. "That is why you need a submersible that has really thick walls."

The carbon-fibre-and-titanium walls of the Titan submersible are designed to give it a maximum operating depth of 4,000m (13,123ft).

Bottom currents

The strong surface currents that can carry boats and swimmers off course are probably more familiar to us, but the deep ocean is scoured by underwater currents too. Although usually not as strong as those found on the surface, these can still involve the movement of large amounts of water. They can be driven by winds at the surface affecting the water column below, deep water tides or differences in the water density caused by temperature and salinity, known as thermohaline currents. Rare events known as benthic storms – which are usually related to eddies on the surface – can also cause powerful, sporadic currents that can sweep away material on the seabed.

What information there is about the underwater currents around the Titanic, which is split into two main sections after the bow and stern broke apart as it sank, come from research studying patterns in the seabed and the movement of squid around the wreck.

Part of the Titanic wreck is known to lie close to a section of seabed affected by a stream of cold, southward-flowing water known as the Western Boundary Undercurrent. The flow of this "bottom current" creates migrating dunes, ripples and ribbon-shaped patterns in the sediment and mud along the ocean floor that have given scientists insights into its strength. Most of the formations they have observed on the seabed are associated with relatively weak to moderate currents.

Sand ripples along the eastern edge of the Titanic debris field – the splatter of belongings, fittings, fixtures, coal and parts of the ship itself that spread out as the ship sank – indicate there is an easterly to westerly bottom-flowing current, while within the main wreckage site, scientists say the currents trend from northwest to southwest, perhaps due to the larger pieces of the wreck, altering their direction.

Around to the south of the bow section, the currents seem particularly changeable, ranging from northeast to northwest to southwest.
Crews are protected from the crushing pressure of the deep ocean by the thick, reinforced walls of their submersible (Credit: Alamy)
Many experts expect the winnowing of these currents to eventually bury the Titanic wreckage in sediment.

Gerhard Seiffert, a deep-water marine archaeologist who recently led an expedition to scan the wreckage of the Titanic in high resolution, told the BBC that he did not believe the currents in the area were strong enough to pose a risk to a submersible – provided it had power.

"I'm not aware of currents representing a threat for any functioning deep-sea vehicle at the Titanic site," he says. "The currents… in the context of our mapping project, represented a challenge for precision mapping, not a risk for safety."

The wreck itself

After more than 100 years on the seabed, the Titanic has gradually degraded. The initial impact of the two main sections of the vessel when it collided with the seafloor, twisted and distorted large sections of the wreckage. Over time, microbes feeding off the iron of the ship have formed icicle-shaped "rusticles" and are speeding up the deterioration of the wreck. In fact, scientists estimate that the higher bacterial activity on the stern of the ship – largely due to the greater level of damage it endured – is causing it to deteriorate 40 years faster than the bow section.

"The wreck is constantly collapsing, mainly due to corrosion," says Seiffert. "Each year a tiny bit. But as long as you keep a safe distance – no direct contact, no penetration through openings – no harm is to be expected."

Sediment flows

Although it is extremely unlikely, sudden flows of sediment along the sea bed have been known to damage and even carry off human-made objects on the ocean floor in the past.

The biggest of these events – such as the one that severed transatlantic cables off the coast of Newfoundland in 1929 – are triggered by seismic events such as earthquakes. There is a growing appreciation of the risk these events pose, although there isn't any indication that an event like this is involved in the disappearance of the Titan submarine.

Over the years, researchers have identified signs that the seabed around the Titanic wreck has been hit by huge underwater landslides in the distant past. Huge volumes of sediment appear to have cascaded down the continental slope from Newfoundland to create what scientists call an "instability corridor". They estimate the last one of these "destructive" events occurred tens of thousands of years ago, creating layers of sediment up to 100m (328ft) thick. But they also happen extremely rarely, says David Piper, a marine geology research scientist with the Geological Survey of Canada, who has spent many years studying the seabed around the Titanic. He compares such events to the eruption of Mount Vesuvius or Mount Fuji in terms of how often they might occur – on the order of once every tens of thousands to hundreds of thousands of years.

Other events known as turbidity currents – which are where water becomes loaded with sediment and flow down the continental slope – are more common and may be triggered by storms. "We show a repeat interval of perhaps 500 years," says Piper. But the topography of the seafloor in the area would likely steer any flows of sediment down a feature known as "Titanic Valley", meaning it would not reach the wreck at all.

Both Seiffert and Piper say it is unlikely that such an event might have played a role in the disappearance of the Titan submersible.

There are other geological features around the wreck site that have also still to be explored. In a previous expedition to Titanic with OceanGate, Paul-Henry Nargeolet – a former French Navy diver and submersible pilot – visited a mysterious blip he picked up on sonar in 1996. It turned out to be a rocky reef, covered in sealife. He had hoped to visit another blip he had detected near the Titanic wreck in the latest expeditions.

While the search for the missing craft continues, there are few clues about what may have happened to the Titan and its crew. But in such a challenging and inhospitable environment, the risks of visiting the wreck of the Titanic are as relevant today as they were in 1986 when the first people to set eyes on the vessel since it sank made the journey to the depths.

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