What Submarine Can go the Deepest

On 26 March 2012, Hollywood film director James Cameron ascended from the deepest part of the deepest oceanic rift in the world: the Mariana Trench, in the western Pacific. He wasn’t the first person to reach the abyssal 11-kilometre (6.8-mile)-deep valley in its floor, the Challenger Deep, and the publicity around the event probably had as much to do with the fact that he is such a well-known celebrity as anything else.

Cameron was actually the third person to go there (after Don Walsh and Jacques Piccard’s 1960 descent in the Bathyscaphe Trieste), but he was part of the second manned mission to the Challenger Deep and the first person to reach the bottom of the Mariana Trench solo. And to put all that into better perspective, NASA alone has sent 24 men to the Moon, 12 of them actually leaving their command modules and walking around on its surface, which would have been an impossible feat for this trio of intrepid aquanauts.

So what are the challenges posed by this geological giant, which could swallow Mount Everest and still leave over two kilometres (1.25 miles) of water above its highest peak? The single biggest obstacle for any submersible diving to these depths is the extreme pressure. Because seawater has more mass than air per volume – typically 1,025 kilograms per cubic metre (64 pounds per cubic foot) versus 1.23 kilograms per cubic metre (0.08 pounds per cubic foot), for roughly every ten metres (32 feet) you dive into the ocean, the pressure increases by one standard atmosphere (one bar). So the pressure near the bottom of the Challenger Deep exceeds 1,000 bars, or 1,000 kilograms per square centimetre (14,500 pounds per square inch), although temperature, tides and other factors mean this varies.

Submarine Can go the DeepestNaturally such extreme pressures would crush us to a pulp, so a manned submersible that visits the Challenger Deep needs to have enormous compressive strength to maintain the habitat inside it, while keeping its human occupants warm and supplying them with breathable air.

Cameron’s Deepsea Challenger had a similar structure to the Bathyscaphe Trieste, though its torpedo shape was designed to descend lengthways. At one end is the pilot sphere, the only line of defence against a wall of deadly water. To minimise weight and increase strength, the interior is just 109 centimetres (43 inches) in diameter, while the hull is made of 6.4-centimetre (2.5-inch) steel. The spherical shape of the chamber makes it much stronger; if it was cylindrical like the rest of the sub, it would need to be three times as thick.

To facilitate its descent, 450 kilograms (1,000 pounds) of steel weights are held on the side by electromagnets. These are dropped when the pilot needs to rise, but in case they don’t (thereby marooning the submersible on the ocean floor), a power failure will drop the weights automatically, the support team on the surface can trigger the command themselves and, as a failsafe, a wire that helps connect the weights to the submersible will corrode and snap after 13 hours’ exposure to seawater.

In any case, the Deepsea Challenger uses syntactic foam floats, dense enough to withstand the pressure yet lighter than water – these are able to rapidly lift the craft back to the surface in just half the time it took to reach the bottom.