Surviving the extreme pressure of deep-sea exploration Justin Marozzi on pushing the boundaries of deep ocean exploration

Half a century after the bathyscape Trieste reached the bottom of the Mariana Trench, Nekton continues to push the boundaries of deep ocean exploration.


Unlike some animals, Homo sapiens have not evolved to survive exposure to the severe pressures encountered at depth. The record human dive on one breath is 214 metres over four-and-a-half minutes, a figure put to shame by Cuvier’s beaked whale, which has been recorded diving to an extraordinary 2,992 metres, holding its breath for more than two-and-a-quarter hours. Animals like us that live at sea level pressure of one atmosphere are highly sensitive to increases in pressure and suffer physiological problems, with neural and muscular functions especially prone to disturbance. Unlike whales, we do not have the ability to collapse our lungs to store oxygen and reduce the absorption of nitrogen.

Submersibles are the technological shortcut and allow scientists and explorers to descend safely into the ocean to study this little understood environment. Since water pressure increases steadily with depth, with each 10 metres adding another atmosphere of pressure on the vessel, by the time you reach the deepest places on earth – the Mariana Trench hits 10,994 metres (considerably lower than Mount Everest is high) – the US National Oceanic and Atmospheric Administration reports that the pressure would be equivalent to having 50 jumbo jets on your head.

Nekton, an ambitious marine expedition in the Sargasso Sea that combines science and exploration to better understand the health of the deep ocean, has to deal with potentially fatal pressures on a daily basis. The team of technical divers descend to around 100 metres to conduct video surveys of the seabed and take samples of coral and sponge. With their complex equipment and rebreathing systems they can remain underwater for six hours, the great majority of which is time spent ascending safely towards the surface, making a series of decompression stops to reduce the excess pressure of inert gases dissolved in the body. Failure to make these stops results in decompression sickness, commonly known as ‘the bends’ and, in extreme situations, death.

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Nekton’s two 3.5-tonne Triton submersibles Nemo and Nomad can descend up to three times deeper than the divers and carry a pilot and scientist in an acrylic bubble. At their maximum depth of 300 metres, the subs are withstanding 30 atmospheres. Inside the bubble, however, the pressure is maintained at the normal one atmosphere encountered at sea level. There are no popping ears as experienced when taking off and landing in an aircraft or even the slightest discomfort, bar the occasional worry that the acrylic bubble might burst, bringing our lives to an instant watery end with the massive shockwave as the sphere implodes.

Kevin, the Canadian pilot of our Triton submersible who counts a dive of the Titanic (depth 3,800 metres) among his professional underwater missions, describes submarines more encouragingly as “the safest mode of transport you’ll ever use”. He explains how, if the control systems and thrusters go down and the electronics fail, the submersible passengers can still rest (relatively) easy. Crank a couple of switches and physics takes over – four vents open, air floods into the four main ballast tanks, the buoyancy is instantly increased and the sub returns safely to the surface.

The one area where a compromise at depth would be fatal is the acrylic sphere – “If the sphere goes, we’re toast”, chuckles Kevin – so the acrylic is seven centimetres thick on Nomad and Nemo and a prodigious 16.5 centimetres in the Triton models that have a maximum depth of 1,000 metres. The use of a sphere as the shape best able to withstand high pressures while providing maximum visibility continues a tradition established by the pioneering American divers and explorers William Beebe and Otis Barton, who set a world record in 1934 by descending to 923 metres in a two-man steel Bathysphere to observe deep-sea fauna in their natural environment.

We have long mastered the technology for submersibles required to withstand the most extreme pressures on Earth – the bathyscaphe Trieste reached the bottom of the Mariana Trench in 1960. The next challenge, apart from extending our limited understanding of the oceans, is to reduce their cost. Nomad and Nemo would set you back an eye-watering $2.2m. Financially speaking, that is a different order of pressure altogether.


This is an excerpt from issue 4 of Avaunt, on newsstands now. Subscribe to Avaunt here.

Words: Justin Marozzi

Illustration: Robert G. Fresson

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