At around 6,000m, we finally descend to the fifth of the five ocean zones: the hadal (hadalpelagic) zone. By this point, we’ve descended through the epipelagic (sunlight) zone: the surface layer of the ocean and the only part where photosynthesis takes place.
When the sunlight zone ends and the mesopelagic (twilight) zone begins at around 200 metres deep, just one percent of the sun’s rays can penetrate so it’s dark and cold and there’s no photosynthesis.
Deeper still and the twilight zone gives way to the bathypelagic (midnight) zone followed by the abyssopelagic (abyssal) zone.
- It’s deep enough to hide Mount Kilimanjaro, contains bone-eating worms and is the world’s largest habitat
- “The crushing pressure can be anywhere between 100 to 400 atmospheres, which would kill a human instantaneously.”
In many parts of the ocean, the abyssal zone ends at the seafloor but there are some sections where the abyss gives way to deep oceanic trenches in what is known as the hadal zone, named after Hades, the Greek god of the underworld.
Perhaps the most famous of these oceanic trenches is the Mariana Trench in the Pacific Ocean where Challenger Deep (the ocean’s deepest point) can be found. It’s around 10,935 metres below the surface.
Perhaps surprisingly, the Mariana Trench was first discovered way back in 1875 during the H.M.S. Challenger expedition – although they believed it was a little over 8,000 metres deep. After all, they were using weighted ropes to take their measurements, rather than the high-tech equipment we have available today.
Just under one hundred years later – in 1960 – humans reached the trench for the first time when oceanographer Jacques Piccard and US Navy officer John Walsh in a deep-sea submersible called Trieste.
It took them nearly five hours to reach the bottom. They spent just 20 minutes on the seafloor before they had to return to the surface, a journey that took them more than three hours.
The film director and deep-sea explorer James Cameron visited the Challenger Deep in the Pacific Ocean on 26 March 2012. At around 10,935 metres down, this is the deepest part of the ocean. It took him two hours and 36 minutes to travel around seven miles to reach the bottom.
He was the third person in the world to reach this point and the first to do this dive solo. During his dive, Cameron spent more than three hours exploring the seafloor.
Despite being miles below the surface, there is life even in the hadal zone. Some of the creatures found here are so weird that they haven’t yet been identified.
The deepest known fish species is the Mariana snailfish: a pale pink fish with translucent skin and a quizzical smile that has been recorded at more than 8,000 metres deep. The organisms living down here are not yet well understood.
Although most people on the planet – and even many marine researchers – will never see this remote part of the ocean with their own eyes, it’s not far enough removed to be safe from human influence.
Sadly, humans are even harming the creatures living so many miles deep in the ocean. Researchers have found pollutants contaminating deep-sea crustaceans in the Mariana Trench and Kermadec Trench in the Pacific Ocean and a plastic bag, among other plastic pollution, at the Challenger Deep.
Top image: an Alicella gigantea collected from the Japan Trench in 2022. Credit: Hadal Zone, CC BY-SA 4.0 https://creativecommons.org/licenses/by-sa/4.0, via Wikimedia Commons
