Once a year, something astounding  happens at Australia’s Great   Barrier Reef.

It lasts barely half an hour.

If you jumped into the water at this very moment,   it’d be like swimming through a snow  globe, hundreds of kilometers across.

But these “snowflakes” are actually packets  of eggs and sperm of coral.

Corals might   look like colorful rocks or undersea  gardens, but they’re actually animals.

A coral is a colony of hundreds of thousands  of tiny individual animals called polyps.

Each of these flower-shaped polyps has a mouth and  tentacles.

Polyps secrete calcium carbonate that   creates their skeleton.

It gives them structure  and anchors them to a rock or the seafloor.

Since they can’t move to find a partner  and mix up the gene pool, most warm-water   corals practice “broadcast spawning.”  But with such a short window to meet up,  they have to sync it just right.

The warming  summer waters cue the right month.

The light from  a waning moon cues the right day, and the setting sun cues the exact minute.

Good luck out there!

These bundles contain the coral's gametes  -- its sperm and eggs.

But the gametes don't   mix in there.

The bundles float  to the surface and burst open.

Sperm search out a new egg.

Only one of these  guys will get in.

Look familiar?

Once fertilized,   it starts dividing and transforms into  this adventurous larva called a planula.

The planula swims through the sea,  searching for a place to settle down.

Chemical and light sensors on its backside  guide the planula to the perfect spot.

It wants what we want: a stable foundation, plenty  of sunlight, and room to grow.

The planula cements   itself into place and morphs into a polyp.

As it grows, it absorbs algae called   zooxanthellae from the surrounding water.

See  these green dots?

They live inside the polyps.

The algae give the coral nutrition and its  brilliant colors.

Then something curious happens:   The polyp clones itself.

It grows copies right  out of its side, that then bud their own clones.

Through broadcast spawning and cloning, corals  create the massive reefs we’re familiar with.

But reefs are in danger, and that’s not  just a problem for the corals.

They’re   vital ecosystems that provide food and  shelter for a quarter of marine life,   like fish, crustaceans and sea turtles.

Climate change is the main culprit.

When ocean waters warm up too much,  stressed polyps expel their colorful and   nutritious algae.

This is coral bleaching.

When reefs die and spawning season comes,   it’s harder and harder for the eggs  and sperm to find each other.

So,   researchers at the California Academy of  Sciences in San Francisco have replicated   the delicate spawning conditions in a lab.

Lights mimic moon cycles, and heaters   simulate the change of seasons.

Their goal is to  discover the best ways to grow corals, so more   scientists can help restore them to the oceans.

An underwater blizzard is a thing of beauty,   even more so when you consider how this snowstorm  can replenish a delicate and threatened ecosystem.