(gentle music) - As the sun sinks towards the horizon here in Costa Rica, one of nature's most spectacular rituals is about to take place.

Hundreds of thousands of sea turtles have come here like a living tsunami.

No one knows exactly what makes these turtles all come to this spot on the same night.

But we do know why.

Over the next few nights, tens of millions of eggs will be laid on this beach.

But tomorrow, locals will come and dig up thousands of those eggs.

Every sea turtle species is either threatened or endangered today, which is why it's illegal to collect eggs basically everywhere in the world.

But here on this beach, it's actually encouraged.

It's hard to believe, but stealing a few eggs might be the secret to saving this sea turtle species.

And we are here to find out how (gentle music) Oh, man.

Oh, getting to these turtles is not easy.

This is more of a full body workout than a road trip, if I'm being honest.

But my journey is nothing compared to the one that the turtles have been on.

I better watch the road.

A few times every year, up to half a million olive ridley sea turtles descend on this seven kilometer stretch of sand.

It's one of only a handful of places in the world where this incredible phenomenon known as an Arribada occurs.

But what exactly are they all doing here?

Here's a hint.

All of these turtles are females, and they're here for one purpose, to create the next generation.

This mass nesting behavior likely evolved as a way to overwhelm predators and scavengers.

There may be plenty of hungry mouths around, but they can only eat a tiny fraction of the eggs laid here.

Yet, despite this spectacular scale, these sea turtles, like all others, face an uncertain future.

Olive ridley sea turtles are the most abundant species of sea turtle on earth today, but their numbers are far below what they were only a century ago.

Sea turtles like these have existed for over a hundred million years.

They even survived the impact that killed the dinosaurs.

But today, their survival is in serious jeopardy.

The biggest threat isn't sharks or storms or scavengers.

It's us, from fishing nets and coastal development to plastic pollution and poaching.

These ancient mariners face challenge after challenge in their journey to hatch the next generation.

But here in Costa Rica is a bright spot for sea turtle conservation.

This country of just 5 million people has come up with a surprising idea for protecting these sea turtles.

This is absolutely incredible.

I have never seen anything like this in my life.

Right now under my feet, I don't know, there could be like four or five more nests just where I'm standing.

What's so mysterious is the unison of this arrival.

These olive ridley sea turtles are solitary creatures in the wild.

They spend years wandering the open ocean alone.

No one knows what signal tells them all to come to this tiny sliver of beach at the same time.

What is it that tells them to come back to the very place that they were born to lay the next generation?

(gentle music) These turtles have been wading off shore for days, maybe even weeks before the Arribada begins.

We know their arrival has something to do with the new moon.

Maybe that darkness protects them from predators.

It could affect their circadian rhythms.

But whatever that cue is, when it goes off, all of these turtles leave the water at the same time, and it is absolutely incredible.

I have to watch my back because she just sprayed me with sand.

(waves whooshing) This turtle, like the thousands of others on this beach, is laying eggs right now.

Each turtle will lay 80 to 100 eggs in the nest that they dig, and it's estimated over the course of this Arribada, up to 10 million eggs will be deposited on this beach.

That abundance of eggs attracts predators and scavengers in droves.

But after they've eaten their fill, they've barely made a dent in the egg supply.

After the first wave of turtles has arrived on the beach and laid their eggs, the beach turns into a scene of utter tragic carnage.

Later arriving turtles inadvertently dig up the nests of the turtles that came before them.

The competition for nesting space is just so high.

The entire beach is littered with destroyed eggs.

(speaking foreign language) - It's estimated that 70 to 80% of the eggs laid early in the Arribada are destroyed by turtles nesting later.

But that gave local conservationists an idea.

If those early eggs were never going to hatch anyway, maybe they could be used to save the rest.

And that is the ingenious logic that created the Ostional Development Association or ADIO.

This community group struck a deal with the Costa Rican government.

They are the only people in the world legally allowed to harvest these protected eggs.

(speaking foreign language) - The collected eggs will be sold as food in local markets.

I can't believe how many eggs are in here, and this is less than 1% of the eggs that were laid.

These are the eggs that will sustain this operation, and it's just a reminder of how many eggs get laid here during Arribada.

It's incredible.

It's mind boggling.

(gentle music) (speaking foreign language) - The locals only take the eggs already condemned by nature's chaos.

That tiny fraction fuels an engine that protects the millions left behind.

Because this community has a literal stake in the turtle's survival, they've become their fiercest guardians.

It's a blueprint for conservation that the rest of the world is watching.

About 45 days after the Arribada, the community's investment pays its first tiny dividends.

These lucky little ones have survived destruction by turtles and predation by predators, but they've got one more challenge ahead of them.

They must scramble for the sea.

It's the most important journey of their lives and also the most perilous.

But here's the most astounding part of this entire life cycle.

The key to the Arribada mystery is locked inside the brains of these silver dollar sized hatchlings.

Remember how we said the turtles were attracted to this beach as if by magnetic force?

Well, some scientists think that tiny crystals of the magnetic mineral magnetite in the turtle's bodies may help the hatchlings sense earth's magnetic field, an imprint on the magnetic signature of this beach and coastline.

This process called magnetic imprinting gives them the knowledge they need to navigate the entire open ocean.

And then a decade or so later, they return to this same volcanic sand to continue this cycle.

The story here in Ostional now is a perfect lesson that conservation isn't always about drawing a hard line between humans and animals.

Here they had a different idea.

They could create a symbiotic relationship between this community and their non-human neighbors.

Ostional challenges us to rethink what conservation means because when local communities take the lead, new ideas and new hope can hatch.

(bright music)