A kilometre from the surface,
beyond the reach of the sun,
a giant black void
larger than all the rest of the world's habitats combined.
There's life here.
But not as we know it.
Alien-like creatures produce dazzling displays of light.
Nearly all animals need to attract mates
and repel predators.
This language of light is so widespread here
that these signals are probably the commonest form of communication
on the entire planet.
And yet we still know little about them.
Hunters illuminate themselves, and by doing so,
attract inquisitive prey.
This is fangtooth.
It has the largest teeth for its size of any fish.
There are precious sensors all over its head and body,
which can detect anything moving in the surrounding water.
It's the midnight zone's most voracious fish.
But prey use light as a distraction,
a decoy of luminous ink.
Down here, in this blackness,
creatures live beyond the normal rules of time.
Siphonophores are virtually eternal.
They repeatedly clone themselves,
some eventually growing longer than a blue whale.
Down here,
it snows.
Continuous clouds of organic debris drift slowly down from above.
This is food,
and a whole variety of filter feeders depend on it.
Jellyfish.
And delicate sea cucumbers.
The 1% of marine snow they miss, eventually settles on the sea floor.
Over millions of years,
it forms a layer of mud up to a kilometre thick.
It's an empty plane that covers
half the surface of our planet.
The deep seabed may at first appear lifeless,
but it's home to a unique cast of mud dwellers.
The sea toad.
It is an ambush predator with an enormous mouth
and infinite patience.
This fish has been living for so long here
that its fins have changed into something more useful.
Feet.
They help it shuffle about on the sea floor.
The flapjack octopus.
It hovers just above the surface of the mud as it delicately sifts through it,
searching for worms.
But it can jet away at the first sign of danger.