How deep is the Ocean? Explore fascinating creatures of the deep.

Long regarded as an empty, desert-like environment with conditions too extreme for life to survive, the deep sea was historically considered insignificant.

However, our outlook on the deep-sea shifted when the HMS Challenger set off to circumnavigate the globe in 1872. It uncovered a diversity of deep-sea life previously thought impossible, and we’ve been making new and exciting discoveries ever since.  

And we’re not finished yet.

According to Ocean Census, we’ve only discovered 10% of Ocean life. It’s estimated that 1-2 million marine species remain undiscovered.

What’s more, a staggering 50% of the Earth’s surface is deep-seafloor below 3,000m. 

We’re constantly finding pieces of the puzzle to improve our understanding of this vast and complex world. From new underwater mountain ranges to previously undescribed species of deep-sea octopus, almost every deep-sea exploration mission yields mind-blowing new discoveries.  

When we consider the scale of the deep sea, we realise that it’s not an unusual habitat at all. In fact, it’s the norm for much of our blue planet, and it’s our land-based habitats that are comparatively rare.

So, how deep is the Ocean?

Let’s dive into the deep Ocean and explore this weird and wonderful world. 

Sunlight zone

We begin our voyage at the surface, in the sunlight zone. This Ocean surface layer extends from 0 – 200m (656 feet) and is where most of the visible light exists.

There’s enough sunlight here for photosynthesis, which forms the basis of the food chain. 

Despite only making up 2-3% of the entire Ocean, the oxygen in every other breath we take is produced in the sunlight zone by photosynthetic plankton (phytoplankton).   

Twilight (Mesopelagic) zone

As we descend below 200m, we enter the twilight zone. Only 1% of sunlight reaches these depths, so light is very faint.

Want to see how light disappears as you dive into the Ocean? Watch this. 

Due to the lack of sunlight, there are no primary producers (organisms that get their energy from sunlight or other non-living sources). Animals that inhabit this zone depend entirely on those living at the surface.

Some scavenge on organic waste material that rains down from above, providing a vital source of nutrition. This “marine snow” largely consists of decaying matter from dead organisms, faecal matter, detritus and other inorganic particles. 

Other marine life undertake a daily mass-migration to and from the deep known as diel vertical migration.

Like commuters making their way into the city, trillions of tiny deep-sea creatures ascend to more abundant waters during the night to feed. They then descend back to the deep Ocean during daylight hours to avoid predators and UV radiation.

This daily surface-to-deep commute is the largest daily migration of life on Earth, and is mostly carried out by zooplankton, krill, and other small amphipods (crustaceans). 

Permanent residents of the twilight zone are adapted to survive in this (almost) lightless world. One of the most notable features are their eyes.  

For example, the cock-eyed squid live between 200-1,000m. These lobsided creatures have an enormous left eye that’s permanently pointed up towards the surface, allowing them to spot the silhouettes of prey against the light from above. 

Midnight (Bathypelagic) zone

At 1000m (3,280 feet), light no longer penetrates, and we’re left in complete, constant darkness.

We have now entered the Ocean’s midnight zone.

It’s cold down here, at a constant temperature of around 4˚C, and the only light comes from the bioluminescence of animals themselves.

In this vast, lightless world, it can be difficult for animals to find food and a mate.

Some extraordinary species have adapted to overcome these challenges in astonishing ways.   

1. Pelican eels have an enormous jaw relative to their body size.  

This can unfold to engulf prey much larger than the eel itself, allowing them to bypass the size-based food web structure (individuals generally only consuming food smaller than their own body size) that usually exists in surface waters. 

2. Cookie-cutter sharks have special suction-cup-like lips and bandsaw-like teeth.  

Their specialised jaw allows this parasitic attacker to attach to much larger animals and gouge out a round chunk of flesh (yes, like a cookie-cutter).  

3. Finding a mate can be equally as challenging: This is how angler fish have adapted. 

Female deep-sea angler fish are famous for their bioluminescent fishing-rod-like lure which extends out from the top of their heads (think *Finding Nemo*). Male variants, however, are much smaller in comparison.

These “dwarf males” spend their lives scouring the darkness in search of a female counterpart. When he finds her, the male latches on to the female with sharp teeth.

This attachment is followed by fusion of the epidermal (skin) tissues, and eventually his circulatory system fuses with hers. He becomes a permanent appendage to her body, in a process known as sexual parasitism.  

A female angler fish can have multiple males attached to her at any one time. 

Abyssal (Abyssopelagic) zone

As we descend below 3,000m (9,843 feet) in the Ocean, we reach the pitch-black bottom layer known as the Abyssal zone. Physical conditions down here are still, with slow moving currents, constant near-freezing temperatures, and bone-crushing pressures. 

Time seems to stand still. There’s no primary production and most organisms depend almost entirely on the marine snow that slowly rains down from above. 

But it’s far from being a lifeless desert: The abyssal zone stretches across wide plains, towering seamounts and hydrothermal vent systems, covering more territory than all of Earth’s continents combined. 

Inhabitants of the abyss move and grow very slowly to minimise energy needs. 

Some individuals have specific adaptations to maximise their chances of survival. Tripod fish have modified pelvic and lower caudal fins which can extend up to a metre.  

This allows the fish to stand on stilts above the seafloor, so that it’s perfectly positioned to eat any small fish or crustacean that come travelling along the currents.  

The slow-growing nature of these abyssal ecosystems means that they take a long-time to recover from any disturbance events (if ever). Consequences of overfishing and proposed deep-sea mining of the abyssal zone will therefore cause devastating, irreversible losses to habitats and biodiversity. 

The Trenches (Hadalpelagic zone)

We continue our journey below 6,000m (19,685 feet) to enter the trenches (hadal zone): The deepest part of the Ocean.  

The hadal zone was once considered unsuitable for the survival of animals. However, the rapid development of exploration technologies has allowed scientists to discover species belonging to many of different taxonomic groups at these depths, including crustaceans, molluscs, and echinoderms. 

The Pseudoliparis snailfish is the deepest known fish. It was discovered close to the very limit of survival for all fish in August 2022 at 8,336m (27,349 feet).  

Its adaptations include a flexible skeleton to tolerate extreme pressure, a gelatinous coating to improve energy efficiency, and a large stomach for opportunistic feeding.  

We continue down to surpass the inverted height of Mount Everest at 8,849m. A further two kilometres on, we reach the bottom of the Mariana Trench and the deepest known point in the Ocean (and Earth):  

The Challenger Deep – 10,935m (35,876 feet)

The weight of all the water overhead here is over 8,000 kg per square inch. That’s roughly 1,000 times the pressure at the surface, and equivalent to 1,800 elephants on top of you! 

In 1960, Don Walsh and Jacques Piccard made history by becoming the first people to reach the bottom of the Challenger Deep in the manned submersible Trieste. This record was broken on 26^th March 2012 when James Cameron made the first ever solo dive to the Challenger Deep in the Deepsea Challenger.

To this day, reaching the deepest known part of the Ocean remains a challenge for any explorer, and every expedition yields new discoveries as well as other, more ominous findings. 

During the Fendouzhe deep-sea expedition in 2020, researchers discovered plastic bags, electric wire, a beer can and fibre-optic tethers among other forms of plastic pollution in the Challenger Deep.  

This shows that even the most remote, hard-to-reach place on Earth is still not safe from human impacts. 

We can’t treat the deep-sea as out of sight and out of mind. More work needs to be done to safeguard these precious ecosystems and ensure that life in the deep doesn’t disappear before we even have the chance to understand it.