It is November.
A full moon rises.
Almost 400,000 kilometers away, Planet Earth falls under its spell.
Our oceans stir.
An ancient undersea ritual is underway.
The outcome of this event
will not only affect marine life in the surrounding waters,
but will also give us a clue
about the health of the entire ocean system.
The creature responsible is coral.
And this is coral's story.
The story begins here,
with the incredible phenomenon that is mass coral spawning.
It's an annual event, cued by the sun and the moon.
It involves over 140 different species,
and has been occurring for thousands of years.
Although corals have no brain and no eyes,
somehow individual species spawn together,
sometimes down to the minute.
Like tiny love-letters,
the genetic material is cast into the ocean currents,
connecting reefs over hundreds of kilometers.
Like us, corals take up to nine months to prepare for birth.
But many are hermaphrodites, so they gestate both egg and sperm.
On spawning night,
these are grouped into bundles and delivered through the coral's mouth.
For most, it's a gentle birth.
Over a short time-span, the bundles break apart
and the egg and sperm must find their counterparts from another colony.
Billions set forth, but few survive.
Many other creatures choose the same conditions to spawn
and the reef waters are transformed
into a vast liquid womb, clouded with eggs and sperm.
Mass spawning overwhelms potential predators,
so the coral eggs have more chance to be fertilized and begin a new colony.
By daybreak, vast slicks on the surface mark their passage.
Coral is an architect not only of the undersea world,
but above the surface too.
Dead coral breaks down into sand
that eventually builds up to create coral cays.
These shores and shallows provide sanctuary -
and important breeding grounds - for a variety of other creatures.
All told, reef-building corals have colonized every tropical ocean,
shaped and protected thousands of kilometers of coastline
and influenced marine evolution for millions of years.
Birds like these noddy terns
rely on coral reefs for food and coral cays for shelter.
Birds help vegetate cays by depositing seeds.
These seeds grow into trees and forests
which in turn provide the birds with nesting sites.
These sandy outposts are frequented by many tropical seabirds,
with some pairs returning to the same nesting sites year after year.
Primitive corals date back over 450 million years.
Modern reef-building corals first emerged around 250 million years ago
and their reefs have provided homes and sustenance
for innumerable species ever since.
Over coral's long history, five mass extinctions have come and gone.
During each, up to 90% of all life-forms perished.
Two of these extinctions
occurred during the rise of modern reef-building corals.
Rather than falter, they somehow survived,
rebuilt their reefs
and transformed the oceans anew.
Today, coral reef ecosystems are important nursery grounds
for many iconic marine animals, such as the sea turtle.
There are seven species of sea turtle
and all are long-lived and slow-growing.
They take between 10 to 30 years to sexually mature and mate at sea.
These marine reptiles are nomadic
and swim thousands of kilometers during their lives.
But strong instinct and mysterious natural forces guide them,
so that when a female needs to lay her eggs,
she returns to the very beach where she was born to do so.
Here, she digs a nest and coral sand becomes the womb.
Hundreds hatch, but only one or two will make it to adulthood.
They can then live for up to a century.
When a turtle hatched 100 years ago,
it would have seen the world's coral reefs in all their glory.
Pre-industrial seas were clean and clear
and corals had enjoyed good conditions for millions of years.
Throughout the Indian and Pacific oceans,
and into the Caribbean and west Atlantic,
reefs were thriving.
The turtle spent its youth exploring these reefs
and witnessed the engineering marvel that is coral.
By harnessing the power of the sun and perfecting recycling,
coral built energy-efficient cities that put many of ours to shame.
Collectively, these cities created vast empires,
some visible from outer space.
Here, the turtle found itself in great company,
for these undersea empires were home to vast numbers of marine species
of every imaginable shape and size
and were crowded, colorful places.
Today, reefs have one of the highest levels of biodiversity
of any ecosystem
and are some of the most biologically important places on Earth.
Together, turtles and corals have witnessed many changes
to their watery world.
They are some of the only beings
to have survived the last mass extinction event
65 million years ago.
Scientists predict that in the coming century,
they will face ocean conditions
as catastrophic as during that last extinction.
Wise old turtles have seen the warning signs for a while.
70% of the world's reefs have already been destroyed
or are seriously threatened.
And all but one of the seven sea turtle species
are classified 'endangered' or 'critically endangered'.
For the first time in millions of years,
the future of both turtle and coral is uncertain.
Many things affect the health of coral reefs,
in particular, sea temperature, sea level and water chemistry.
Reefs may look mighty, but corals themselves are fragile
and can be fatally harmed by just subtle changes in their environment.
When sea levels rise too rapidly, corals can't grow fast enough
and reefs drown.
As atmospheric carbon dioxide levels increase,
ocean acidification intensifies and reefs will dissolve.
And a sustained increase of only 1-2 degrees in ocean temperature
can cause corals to bleach and die.
By the end of this century, it's predicted that the global climate
could be 5 degrees Celsius hotter or more.
At this rate, bleaching will become an annual occurrence,
and this colorful landscape will literally fade away.
A healthy coral reef is easy to spot.
A complex structure awash with color, variety and activity.
This is a tangled, tropical garden to rival anything on land.
Most amazing about this garden
is that many of the 'plants' are actually animals.