If the 1960s were all about reaching for the stars, the great adventure of the 2010s could be taking the ultimate plunge.

In 2012, film-maker and ocean explorer James Cameron descended 11km (7 miles) to the bottom of the Mariana Trench in the Pacific Ocean. Though he was not the first to reach this impressive depth, his name was added to the Guinness book of world records alongside US Navy Lt Don Walsh and Swiss oceanographer Jacques Piccard who first ventured there in 1960.

Cameron’s descent marked a resurgence in interest in the depths, particularly the Mariana Trench – the birthplace of many devastating earthquakes and home to Challenger Deep, the deepest part of all the world’s oceans. Thanks to increasingly advanced submersible vehicles and cameras, scientists are discovering more about our planet and the creatures living here.

The video footage from Cameron’s mission revealed there was indeed life at the very bottom of the ocean in the shape of wood-eating crustaceans, camouflaged sea cucumbers and single-celled lifeforms that catch food in their sticky filaments. Yet these creatures have never been seen outside of this pitch-dark environment where they are perfectly adapted to the harsh conditions. So, not all species familiar with the depths can be described as divers.

During this intensive period of exploration, there has also been plenty of competition to find the deepest living fish. In 2014, researchers from the University of Hawaii, US, and the University of Aberdeen, UK, teamed up to use an innovative lander to record video footage in the Mariana Trench. At 8,145m (26,722 ft) a pale pink snailfish was attracted to their bait and welcomed into the record books.

Three years later, Japanese scientists working with the national broadcaster NHK filmed another snailfish 85 feet deeper in the Trench. Snailfish (Liparidae) are widely distributed, but poorly understood, particularly the species encountered at such extreme depths. The deep-dwelling fish have a limited range, with different species discovered in different trenches, and they are described as ‘benthic’ meaning they have a close relationship with the seabed. Based on the few specimens that have been caught, scientists predict it is unlikely any snailfish will be found living deeper than 8,200m below the surface. According to a study published in 2014, this is a biochemical limit, beyond which fish lack the ability to balance out the destabilising effects of pressure on their bodies.

It’s a balancing act scuba divers are all familiar with. In 2015, Ahmed Gabr achieved a record-breaking scuba dive in the Red Sea at Dahab, Egypt. The special forces officer and diving enthusiast plunged 332.35m (1,090 ft 4.5 in). While it took him 12 minutes to descend, he spent almost 15 hours returning to the surface. The ascent is always the riskiest part of a scuba dive due to the possibility of decompression sickness (DCS) if a diver cannot balance the gases in their body. As pressure builds on the descent, nitrogen dissolves into the tissues of the body so if an ascent is made too quickly the gas forms bubbles affecting everything from the joints and skin, to the heart and brain.

This is why ultra-deep dives are not achievable for humans, but there are still some people pushing the boundaries of human endurance to find out just how deep we can dive on a single breath. In 2016, the world record for free-diving was broken by William Trubridge. He plunged 122m (400ft) into the ocean depths, breaking his own record. There are numerous different categories in this most extreme of sports, with divers descending by sled, paddling fins or unassisted like Trubridge. Herbert Nitsch, the “deepest man in the world” holds records across the disciplines, but one attempt in 2012 was likely his last when he blacked out and suffered DCS on the return from a sled-dive to more than 240 m (800 ft). He is still recovering from the life-changing injury to his brain, but he has returned to free-diving, albeit not competitively.

Cuvier's beaked whale (Credit: Andrea Izzotti/Shutterstock)

When it comes to mammalian free-divers, the cetaceans beat even the best of us fins down. In the lead are Cuvier’s beaked whales (Ziphius cavirostris) that can dive from the ocean’s surface down to 2,992m (9,816ft) thanks to adaptations that help them conserve oxygen and survive extreme pressure. These elusive whales aren’t competing for glory, but hunting for deep sea squid.

Stephanie Bush, a research associate at the Monterey Bay Aquarium Research Institute, has a particular interest in cephalopods of the deep sea. Yet when asked about deep sea divers, she still thinks of mammals first.

“When I think of deep diving I think of marine mammals like elephant seals, beaked whales, sperm whales… I don’t think of cephalopods as divers much at all,” she says. “There are many deep-sea squids that move hundreds of meters shallower then deeper over the course of the day, but that would be considered movement or migration, not diving. Diel vertical migration is the term, and loads of other animals do it too.”

It’s bewildering perhaps that while the ocean represents an extreme environment for us, many of its inhabitants regularly travel vertical distances that would take our breath away. Multiple species make trips in search of food every day, spending daylight hours up to 1000m deep and rising up as far as 800m closer to the surface at sunset. In this mass marine commute, fishes, squid and zooplankton alike travel to feed in plankton-rich waters at night when there’s a reduced risk of large predators. They then return to the safer, darker depths at sunrise.

Yet squid certainly make the shortlist of the ocean’s most energetic divers, according to a study by a team at Stanford University, US. Julia Stewart described how they tagged Humboldt squid (Dosidicus gigas) and tracked them rapidly diving to and from depths of 1,500m, keeping up a remarkable pace of 1m per second despite the lower oxygen levels at this depth. Of the cephalopods, Dumbo octopuses (Grimpoteuthis) are currently thought to be among the deepest-dwelling. Species have been discovered between 400m and 4,800m below the surface of the waves inviting further study into how these rarely glimpsed gelatinous creatures live in such varying depths.

This flapjack octopus lives in the deep waters off California (Credit: BBC 2017)

Unlike many fish, which have air-filled sacs known as swim bladders to aid buoyancy, cephalopods have no air in their bodies. This is undoubtedly an advantage for a diver, but we are still learning about the myriad other adaptations this unique class of animals have evolved to survive in varying pressure, temperature and light levels.

While submersibles and tagging technology have helped reveal some of the secrets of the deep, when it comes to defining the ocean’s greatest divers, it seems we still have a long way to go.

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By Ella Davies

Featured image by superjoseph/Shutterstock