Pardon the pun, but it’s almost impossible to fathom just how deep our seas and oceans are. Sure, advancements in oceanographic technology mean that computers can tell us how deep the ocean is, but the fact that 95% of our deep seas remain undiscovered tells you a lot about how mysterious and inhospitable these waters really are.
Don’t worry, though, because, like you, we were keen to get to the bottom of just how deep our seas and oceans are. So, we set forth to find out, leaning on our own knowledge of the deep blue – along with the help of a few trusted experts – to answer the question of how deep the sea is once and for all.
Join us as we take a dive into the mysteries of the deep ocean, as we plunge to the depths to discover the creatures of the deep and the uncharted waters in which they live.
How deep is the ocean?
The fact that we know more about space than we do about the bottom of the sea is evidence of how deep the ocean really is. But how deep is deep? Wrap your eyes and ears around our visual guide to the deep blue below to find out…
See – we told you the sea was impossibly deep. Of course, there’s more to it than just a number, so join us as we take a closer look at the different zones that you’ll pass through on your way down to the murkiest depths of the ocean floor.
The shallow end
Humans have been exploring beneath the waves for decades and yet the deepest we can safely go is only 100 metres – a fraction of the distance it would take to reach the deepest part of the sea. That’s with the very latest diving equipment too.
The official diving limit for scuba swimmers is 100 metres and this is a restriction that’s recognised all around the world. Why? Well, it’s all to do with atmospheric pressure, which increases the deeper you go.
With every 10 metres deeper under the water, the pressure rises by one atmosphere. This means that at 50 metres the pressure is five times higher than it is on the
surface. For that reason, most scuba divers only swim as deep as 60 metres, as after that, specialist equipment is needed.
Given that we can only safely dive to a depth of around 100 metres, that leaves a lot of ocean completely inaccessible to humans. Sure, we can now reach the seafloor in submarines and submersibles, but we’re very much shallow-end dwellers in the grand scheme of things.
So, what of the other zones in the ocean? The ones that we humans could only reach using specially adapted vehicles? Let’s take a look…
Let’s dive into ocean zones
Since the ocean is so deep, scientists created distinct zones to make it simpler to categorise oceanographic conditions based on water depth, along with the animals that live in each unique zone. There are five oceanographic zones in total, including…
Sunlit Zone
The sunlit or euphotic zone is the upper layer of our seas and oceans. As the name suggests, this is the area that sunlight penetrates, roughly to a depth of around 200 metres – about the same as Canary Wharf in London.
Although the sunlit zone only accounts for around 2-3% of our seas and oceans, it hosts the biggest variety of life. That’s simply down to the fact that this zone receives light, making it possible for organisms, plants, and animals to survive. Indeed, most of the animals at Bristol Aquarium live in the sunlit zone, from the red lionfish (Pterois volitans) to the common octopus (Octopus vulgaris).
Twilight Zone
Nope, we’re not talking about an American TV series; the twilight zone is the common name given to the ocean’s mesopelagic layer, which occurs at a depth of around 200 to 1,000 metres. Here, any trace of sunlight disappears, with the occasional flash of bioluminescence from living organisms being the only light source available at this burgeoning depth.
Despite the twilight zone being devoid of sunlight, it hosts a surprisingly diverse array of life – from the tiniest organisms to some of the largest marine species on the planet, including the mighty giant squid (Architeuthis).
Midnight Zone
The midnight zone or bathypelagic layer represents the very bottom of the ocean in most regions around the world. It’s categorised as water from 1,000 to 4,000 metres, where any residual light that may be present in the twilight zone is almost universally extinguished, creating perpetual darkness.
Since this is one of the largest layers of the ocean, it’s no surprise that the midnight zone accounts for up to 70% of all saltwater on Earth. And since it’s so deep and so dark, very little is known about the animals and organisms that live here, with all but the most advanced submersibles able to cope at such depths.
Abyssal Zone
You guessed it: the word “abyss” is derived from this mysterious ocean layer, which scientists categorise as the space at a depth of 3,000 to 6,000 metres. This is the ocean floor in the deepest parts of our seas, where vast underwater plains, mountains, and deep chasms lie in absolute darkness.
Little is known of the abyssal zone, with over 95% of it completely unexplored. Here, the atmospheric pressure can reach up to 600 times that of the surface, which makes exploration all but impossible. Still, life exists here, with remote-operated vehicles (ROVs) capturing evidence of life at the very bottom of the sea – from the cusk eel (Ophidiidae) to the vampire squid (Vampyroteuthis infernalis).
Hadal Zone
The hadal zone is derived from Hades, the Greek god of the Underworld – a fitting name for an area of the ocean that lies at a staggering depth of 4,000 to 6,000 metres. Also known as the hadalpelagic layer, this area of the ocean is only found in a handful of regions across the globe, where deep underwater trenches plunge for thousands more metres into what feels like the very centre of the Earth.
It’s thought that all the trenches under our seas make up an area roughly the size of Australia, with the deepest, the Mariana Trench, descending to an extraordinary 11,000 metres. Remarkably, there are signs of life at these incredible depths, with scientists discovering rare habitats on the steep slopes of the trenches.
Famous dives
Although we realised very early on that the deep sea is inhospitable, it hasn’t stopped some intrepid divers from testing the limits of human endurance. For decades, divers have been in a race to venture deeper than their contemporaries, and some seriously impressive records have been set in that time.
In 1947, for instance, Maurice Fargues reached a depth of over 120 metres, carving his signature in a rock before losing consciousness and, sadly, dying. The reason for his dive was to establish the safe diving limit for future divers, with 100 metres later established as the safest maximum depth for scuba diving.
As for the deepest dive ever made by a single scuba diver, that record belongs to Ahmed Gabr, who completed a dive of 324 metres in 2015 – that’s three times the recommended dive limit and about the same length as the Eiffel Tower!
We hope you’ve enjoyed this deep-dive discovery into the remarkable depths of our oceans. If you’d like to see some incredible creatures of the deep without donning your scuba suit, our Ocean Floor exhibit is the place to do it.