And yet, what we know about it barely scratches the surface.

Beneath its swell is a largely unexplored universe,

until recently, beyond the gaze of human eyes.

So why do we know so little about the ocean?

For a start, immense pressure presents huge challenges

for divers and equipment alike.

In many ways, it's easier to send a mission to space.

But with new technology, such as submarine robots,

this hidden realm is starting to reveal its secrets.

So, what's down there?

Well, there's water. Lots of it.

1,419,120,000 cubic kilometres,

to be about as precise as you can be.

And in that water, there's fish -

the main source of protein for around three billion people.

But there's a lot more than just fish down there.

Extraordinary, otherworldly creatures dwell in the depths,

with new ones discovered all the time.

Many are gelatinous -

jellyfish that disintegrate if you try to catch them in a net.

In 2020, scientists found the giant Siphonophore Apolemia,

an organism made up of millions of interconnected clones,

its thin twisting body reminiscent of a long piece of string.

And the ocean floor is far from being the flat and featureless seabed

you might imagine.

If you were to drain the ocean,

a landscape would emerge just as spectacular as anything on land -

boasting some of the highest peaks,

deepest canyons and longest river channels on the planet.

There are even waterfalls under the sea -

the largest being the Denmark Strait cataract.

Here, the cold waters of the Greenland Sea

meet the warmer waters of the Irminger.

As the cooler water is forced down,

it creates a giant three and a half thousand metre drop,

undetectable to anyone who might be bobbing about on the surface.

And that's nothing compared to the chilling 11,000 metre drop

to the bottom of the Mariana Trench - the deepest place on Earth.

It was here that, in 2020, scientists made an alarming discovery.

At a depth of around 7,000 metres,

in one of the most remote and inaccessible crevices on Earth,

they came across a new species of crustacean.

And it had plastic in its stomach.

They called it Eurythenes plasticus -

a living reminder that,

even though we've barely begun to explore the ocean,

our impact on it is already being keenly felt.

In fact, by 2050,

it's estimated there could be more plastic in the sea than fish.

But it's not just plastic that's a problem.

There are also “dead zones”,

areas with insufficient oxygen to support marine life.

These are becoming more common thanks to pollution.

The sad truth is, when it comes to the ocean,

the reach of human activity

goes far beyond the reach of our knowledge.

It's easy to feel detached from the ocean -

particularly if you live inland.

And this might explain why we've treated it as a dumping ground.

But the more we explore, the more we find it has to offer.

For example, the gene pool of deep ocean life,

such as sponges and microorganisms,

could hold the key to solving the urgent problem

of antibiotic resistance.

More importantly, the ocean is key to almost all life on the planet.

Half the oxygen we breathe comes from marine photosynthesizers

such as phytoplankton and seaweed.

The ocean also regulates our climate,

mediating temperature by distributing solar heat around the planet.

We may not feel it, but every one of us is affected every day

by the role the ocean plays in our finely-balanced Earth system.

And yet, the efforts we've made so far

to protect and preserve this vital life source

are, well, a drop in the ocean.

There's still so much we don't know -

so many breathtaking canyons unseen, so many creatures undiscovered -

but new technology is revealing more about our ocean than ever before.

Perhaps if we knew more of the ocean's secrets

we might look after it better.