♪♪ NARRATOR: In 2013, the Caribbean Sea gave up an extraordinary secret.

It was a vast, pristine coral reef rising up from the Caribbean's deepest waters.

It was the discovery of a whole new world.

PETERSEN: It's just humongous.

It's like a medieval fortress, like big and robust.

HEYMAN: I have never seen any place with the kind of live coral cover, the density of fish.

It was so magic.

NARRATOR: But once discovered, the challenge was how to keep this underwater Eden safe and how to learn from it.

HEYMAN: I realized we needed to research it, learn more about it, understand it better.

But we needed to keep it quiet.

NARRATOR: And so began an undercover scientific adventure, a dive into a living treasure whose secrets may hold the keys to saving many other reefs in trouble.

♪♪ ♪♪ ♪♪ ♪♪ NARRATOR: Fishermen are known for tall tales, but this one is true.

♪♪ ♪♪ It begins in May 2013, when a fisher from Guatemala is pushed into an act of desperation.

♪♪ The big fish he depends on -- the groupers, snappers, and jacks -- have virtually vanished from waters that once teemed with life.

♪♪ ♪♪ He takes a dangerous gambit and pushes farther and farther into the open ocean.

♪♪ ♪♪ As the sun rises, his engine struggles under the strain.

[ Motor sputters ] ♪♪ ♪♪ Miles from land, the water is unexpectedly shallow.

♪♪ Just beneath the swell, a massive crown of rock and coral rises up -- an uncharted, undiscovered reef.

♪♪ It's a tantalizing treasure, a Caribbean secret that will raise hopes for the survival of coral reefs everywhere.

♪♪ The fisher tells only a handful of people.

Ana Giró, Kenny Martin, and Dr. Melanie McField have spent years studying Caribbean corals.

Ana is among the first to hear.

PETERSEN: So, this fisherman comes up to me, and he told me about some rocks.

And so I told him, "Well, take me there.

You know, I want to see the rocks that you -- that you're telling me."

♪♪ ♪♪ It's just humongous.

♪♪ It's like a medieval fortress, like, just big and robust.

♪♪ We have pinnacles that come out.

♪♪ Deep walls.

And then, incredible barrel sponges.

♪♪ It's just mesmerizing.

♪♪ ♪♪ ♪♪ NARRATOR: This newfound reef is a stunning contrast to many other Caribbean reefs that are in peril.

♪♪ It's like stepping back in time to when all reefs were thriving.

♪♪ PETERSEN: There are shallow reefs, vibrant and beautiful and different kinds of fish.

The ecosystem is just so diverse, many types of different communities that compose this incredible system.

♪♪ That was just an incredible experience for me, you know, personally.

♪♪ McFIELD: When Ana told me she found this new reef, that it's just totally amazing, I believed it, but I wanted to see it.

We have benthic habitat maps.

It's not there.

It's not there, you know, when you look on Google Earth, You know, we saw nothing on our maps.

♪♪ NARRATOR: By 2017, the reef's significance is starting to emerge.

It's an underwater mountaintop bigger than Manhattan, crowned with coral and set on the edge of the Cayman Trench, the deepest part of the Caribbean.

It became known as the Cayman Crown.

It's part of a Mesoamerican Reef system straddling the waters of Belize and Guatemala.

♪♪ News of the coral wonder is quietly shared with fisheries expert Dr. Will HEYMAN.

HEYMAN: I was at a meeting and somebody came up to me and told me this incredible story of a place that nobody knew about.

There's giant, giant fleets of ships that go through here supplying the country of Guatemala, the country of Honduras.

But when you check it out, you zoom in, you look at it, all the ships are going this way or they're going this way.

There's a shallow area right between these two massive shipping routes, and that's the Cayman Crown.

It's hiding in plain sight.

What I heard was these were some of the healthiest coral reefs in the world.

I have never seen any place with the kind of live coral cover, the density of fish.

It was so magic.

I realized we needed to research it, learn more about it, understand it better, but we needed to keep it quiet.

NARRATOR: To build the scientific case for protecting the Crown, the team works quietly.

If news got out, fishers might be tempted to target the reef.

PETERSEN: So, we kept it a secret.

So research, the explorations, and everything, kind of just between a selected group of people.

Before we have it, like, protected, that was, you know, our main -- our main goal.

NARRATOR: But protection will only be possible with the support of local fishers.

The team invites Kenny Martin into the circle of trust.

MARTIN: We grew up from a fishing family in a coastal community catching fish on hand lines, and we used to use gillnets.

It has caused a negative impact, and I've seen it now that I go out scuba diving.

NARRATOR: Kenny wants to protect the fish that many people depend on.

That means protecting the reef.

MARTIN: ...1, go.

NARRATOR: Without a healthy reef, there will be no fish to catch.

♪♪ A coral reef is a complex web of life, and different animal and plant species have different roles.

♪♪ Deep gullies lead to an extraordinary coral city.

♪♪ The inhabitants are strange and beautiful and dependent on one another.

♪♪ Parts of the reefscape can feel almost industrial.

The chimney of a single sponge can filter 2,500 gallons of water a day.

There are nurseries for raising young.

And dormitories for the night shift.

♪♪ Everything is interconnected.

At fish grooming stations, spritely cleaners pick off parasites like sea lice.

The reef is also home to giant predators, like this 400-pound goliath grouper.

The gutsy little cleaner is not intimidated.

There are visitors.

A pod of dolphins feeds around the reef.

[ Dolphins squeaking ] Whale sharks, the biggest fish on the planet, gorge on an abundance of tiny zooplankton and fish eggs.

♪♪ Nurse sharks thrive here, too.

♪♪ In a healthy system, predators only take a proportion of their prey.

If the balance is right, the number of predators and prey can be impressive.

♪♪ Predators, like jacks, snappers, and groupers, are vital to the health of the reef and a key food for local people.

♪♪ Most prized by fishers is the Nassau grouper.

Nassau groupers were once the backbone of the local economy, but now they are critically endangered.

♪♪ Groupers hunt at dawn and dusk.

They lie in ambush for small fishes in the reef's darker recesses.

The Nassau grouper, like everything here, depends on a healthy, balanced system.

♪♪ The food web that feeds the groupers starts with sunlight and nutrients.

Currents sweep in microscopic life that's harvested by fan corals and soft corals.

♪♪ Feather duster worms unfurl to collect the invisible morsels.

♪♪ The next level up are grazers, nibbling algae off coral and sponges.

♪♪ French angelfish enjoy a side of sponge with their algae.

♪♪ Parrotfish tend the coral garden.

♪♪ Porkfish forage for mollusks and worms.

♪♪ Everything from microscopic plant to predator is bound together.

Spurred on by the riches, the team has been painstakingly creating an inventory of what lives on the reef and where.

PETERSEN: It took us a long time to explore all the reefs that we surveyed, like 90 kilometers square of the reef.

Doing monitoring of coral-reef health and the geomorphology, and the coral cover was incredible, like nothing that I've ever seen before.

NARRATOR: Ana photographs the coral in strips, building a mosaic of the richer parts of the reef.

♪♪ The photo maps reveal that the Crown has the highest live coral cover in the Mesoamerican Reef.

McFIELD: So we can zoom in right here.

PETERSEN: Yeah, if you zoom in right here, you know the coral composition.

Coral cover, as well.

McFIELD: This is the site with 77% live coral cover?

PETERSEN: Yes, this is the site that we have.

McFIELD: And you see it.

PETERSEN: A huge amount of coral cover.

I mean, compared to the rest of the Mesoamerican Reef, when we're talking about a 19% coral cover... McFIELD: This is the highest now.

PETERSEN: This is -- yeah, this is the highest.

NARRATOR: What is revealed is a coral reef of global importance.

But the team fears that time may be running out.

Caribbean waters are warming alarmingly, posing a grave threat to the corals.

They place temperature sensors on the Crown.

McFIELD: We do have bleaching events in the whole Caribbean, the whole world, obviously.

That is probably one of the greatest threats in this area.

PETERSEN: In October, the water is just so hot, you can feel the heat when diving.

McFIELD: 1998 was the first global mass bleaching event, and it has gotten worse in the last few years.

♪♪ NARRATOR: Tucked within the stony reef are tiny coral polyps.

Inside them, algae make sugars.

But if it gets too hot, the algae make toxins.

♪♪ Then the polyps eject the algae, and the coral bleaches white.

The algae can return if the water cools soon.

Otherwise, the coral often dies.

♪♪ From 2014 to 2017, a bleaching apocalypse killed up to a third of the world's corals.

♪♪ Amazingly, the Crown was unscathed.

But in October 2019, tragedy strikes the region.

♪♪ The Crown takes a direct hit.

♪♪ PETERSEN: It was completely bleached.

Like, all of it.

It was just all white.

HEYMAN: We saw a massive bleaching event.

I mean, the corals that we came to know and love blanketed with snow, it looked like.

♪♪ NARRATOR: Then, four months later, the team notices something extraordinary.

PETERSEN: When we go back in January, the corals get better.

February, they get even better.

♪♪ May, the corals are looking fine.

They're looking beautiful.

♪♪ McFIELD: We have this beautiful Cayman Crown Reef, and it has recovered.

The corals in this region are resilient.

♪♪ NARRATOR: The news reaches coral expert Myles Phillips.

He joins the team to investigate the Crown's remarkable recovery.

Will Heyman shares his theory.

HEYMAN: Let me give you the geographic context.

So, this is the Gulf of Honduras.

PHILLIPS: Yeah.

HEYMAN: And in this channel, that's your Cayman Crown.

What you got to remember is that this is the Cayman Trench.

Here, it's already a thousand, 1,500 meters, and then drops, drops, drops, drop, drops -- 7,600 meters, deepest point.

♪♪ NARRATOR: At its deepest, the trench is over 4 miles down.

A 15-pound bowling ball dropped in would take over an hour and a half to hit bottom.

♪♪ Cold currents rising from those depths sweep nutrients up to the Crown.

♪♪ HEYMAN: This water comes slamming up onto this vertical shelf.

So where we were, those jewel reefs -- PHILLIPS: Right at the top of it.

So that's the source of your upwelling.

HEYMAN: Exactly.

♪♪ NARRATOR: The upwelling flows along folds of rock and coral that act like pipes and channels in a cooling system, easing temperatures and providing nutrients.

♪♪ HEYMAN: The other factor that we haven't talked about, we get 4 meters, like 12 feet of rain, that falls all within a -- PHILLIPS: All in this region.

HEYMAN: Through agricultural lands, through -- PHILLIPS: Into this one bay, through the same little channel.

HEYMAN: Exactly.

PHILLIPS: So this is forming, like a -- like, a roof, like a shade over this coral reef.

HEYMAN: That's it.

NARRATOR: Just as the Caribbean water temperature peaks, muddy water washes over the reef.

Emerging research suggests that this runoff could shield corals from damaging sunlight.

♪♪ HEYMAN: They're getting shade, they're getting fed, and they're getting cooled.

So where all the rest of the Caribbean took that bleaching and wrestled coming back, this area is resilient because of those factors.

NARRATOR: Another factor may be the corals themselves.

Myles finds an abundance of slow-growing corals that can withstand warming waters.

He also sees a surprising amount of faster-growing corals that proliferate like weeds after bleaching events.

PHILLIPS: Well, what I found was lettuce coral, considered the poster child for these weedy species that, honestly, reproduce so quickly or so effectively that they can quickly repopulate an area.

You also had these stress-tolerant corals that invest a lot of metabolic energy in surviving things -- big brain corals.

And they were massive at the Cayman Crown.

NARRATOR: But what astounds Myles is something he doesn't find.

A deadly disease has been sweeping the Caribbean.

To Myles' relief, the normally susceptible pillar and brain corals seem untouched here.

PHILLIPS: I saw no sign of stony coral tissue loss disease.

Actually, no sign of any disease at all that I could see.

These corals must be tens or hundreds of years old, and they look like they've never gone through anything in their lives.

They're completely healthy.

NARRATOR: The crown has been a sanctuary, except its isolation has not protected it from another invader.

♪♪ It's a creature that's become a global menace.

Lionfish.

They were first seen around Belize in 2008.

♪♪ These predators from the Indo-Pacific are among the most aggressive, invasive species on the planet.

♪♪ ♪♪ Even the regal Nassau grouper is wary of the lionfish's venomous spines.

♪♪ This disturbing discovery shows that the Cayman Crown is not totally disconnected from the outside world.

The reef has trade routes.

♪♪ Fish use highways reaching out way beyond the reef.

♪♪ The Crown, the team realizes, could be part of a network stretching across the Caribbean with the kind of connections that we are just beginning to understand.

But untangling these connections has to be put on hold.

[ Thunder crashes ] [ Wind howling ] [ Thunder rumbles ] [ Thunder crashes ] Hurricanes and other storms in the Caribbean are becoming more frequent and powerful, most likely due to climate change.

The researchers can only hope that the Crown's location will protect it as a storm lashes nearby shallow reefs.

McFIELD: What we're seeing in areas that are being hit over and over with the intensity of hurricanes is that that is one of the main causes of reef decline.

And we don't have that down here.

It's very minimal.

The Cayman Crown's perfectly situated below the main hurricane belt.

NARRATOR: The Crown's exact position is yet another factor that may make it a kind of ark, able to seed other reefs with life.

♪♪ After the storm, the team is back at work documenting the Crown's inhabitants.

♪♪ Hamlets are easily overlooked fish, but seeing the start of their courtship brings renewed hope.

♪♪ ♪♪ They release hundreds of microscopic eggs into the current, which sweeps them away from the Crown's hungry mouths.

♪♪ A sea cucumber also adds eggs into the current.

♪♪ A school of blue tangs joins the party at a favorite parapet.

The females release eggs, the males sperm, and the eggs are fertilized.

♪♪ Currents can carry eggs and larvae hundreds of miles, seeding reefs from Cuba to Honduras.

♪♪ Others linger in gyres around the crown.

♪♪ From a few days old, larval fish can swim, guided by temperature, currents, or maybe even sound.

They are heading for the mangroves on a nearby island.

It's an in-between world of marine trees that will serve as their nursery.

♪♪ ♪♪ There are gangs of pint-sized predators, like snappers and jacks, learning the ropes.

♪♪ An inch-long barracuda lays low.

♪♪ Here's a baby cubera snapper, a miniature version of the voracious hunter it will become.

Young wrasse go to school.

And joining the class are baby butterfly fish.

♪♪ The next stop for many young fish is seagrass meadows.

They find more food here but have less protection than when hiding in the mangrove roots.

♪♪ Young sardines try to avoid run-ins with Spanish mackerel.

♪♪ ♪♪ The abundance of life attracts nurse sharks and sting and eagle rays that scour the shallow beds.

♪♪ The juvenile fish will set out for new reefs when they're old enough.

♪♪ To the researchers, the reefs, the seagrass meadows, and the mangroves make up a trinity of life, each an essential part of a single system.

PETERSEN: For snappers and groupers, we need to preserve all of it.

We need to preserve the mangroves.

We need to preserve the seagrass beds and the coral reefs.

The connectivity is really important.

♪♪ NARRATOR: Young fish come back to the Crown, the hub of the system.

Many fill the ranks of grazers or cleaners, ever vigilant for predators also coming of age, like these snappers.

♪♪ The journey to adulthood can last several years.

But snappers can live to be 80.

♪♪ Each year, the predators aggregate at breeding grounds on the reef.

♪♪ ♪♪ Finding the location and timing of breeding big fish is like winning the lottery for a fisher.

Spawning sites are targeted all across the Caribbean.

MARTIN: We've been seeing a lot of fish being caught and most fish with eggs.

We've noticed that, every time you go out, you'll find less fish.

NARRATOR: Kenny and Will are searching the Crown for likely fish-breeding sites to protect the spawning when it happens.

Both believe that the Crown must have major breeding grounds for Nassau groupers and other species based on the currents and ideal rocky outcrops and spurs.

HEYMAN: These fish all come together to reproduce at these very specific places and times at elbows.

This stage is promontory stage, so it's kind of like a table corner, right?

Dropping off on two sides.

NARRATOR: Multiple species can gather at a single site.

HEYMAN: What blows me away about these multi-species aggregation is, they all come at slightly different times.

They're all queued up with the lunar cycle, with the moon, and with the seasonal cycles.

I want to understand and characterize these aggregations.

We've got to know what's going on in order to really protect it.

NARRATOR: Will and Kenny zero in on the most likely outcrops and edges.

HEYMAN: I can't be there all the time, so we use remote sensing.

NARRATOR: They place cameras and microphones to collect data during the breeding season.

HEYMAN: That data's not easy to analyze.

It takes time.

And time is not on our side.

♪♪ NARRATOR: They also call in an expert on grouper spawning aggregations.

Dr. Michelle Scharer studies fish acoustics.

SCHARER: My main interest is in sound production of the fishes.

They live in this environment that is full of sound, but very few people actually have studied that.

There's an opportunity there to learn a lot about coral-reef ecology just by listening and not so much seeing what's going on.

Took a little bit of training to hold your breath and actually listen when you're down there to the sounds and the different pitches and the different rumbles that are underwater.

[ Discordant scratching ] [ Low-pitched thrum ] NARRATOR: What sounds like a cacophony to us... [ Scraping ] ...is actually a chorus of communication.

SCHARER: Fish can definitely hear.

They have ear bones just like we do, where they can actually sense those vibrations in the water.

[ Low-pitched thrumming ] Basically, fishes are communicating.

They are emitting signals and receiving signals that let them know what's going on, if they're ready to spawn.

[ Thrumming ] NARRATOR: The scientists collect an underwater recorder set up months ago.

SCHARER: We're recording 24 hours, and there's sounds accompanying all those moments.

[ Croaking ] It sounds spectacular.

Just the amount and diversity and the frequencies that we're hearing.

[ Croaking ] And there's other little grunts and beeps and boops that are different organisms that live on the reef.

So I think it sounds pretty cool.

Different species will have different frequency bands.

[ High-pitched croaking ] -Toadfish.

-Toadfish, oh!

[ Laughs ] NARRATOR: Toadfish are the noisy neighbors.

Squirrelfish communicate in clicks.

[ Squirrelfish clicking ] [ Scraping ] Grunts make sound by grinding their teeth.

[ Scraping continues ] A batfish bleats.

[ Batfish bleating ] Michelle has identified fish calls used to court, defend territory, and warn of danger.

[ Low-pitched thrumming ] Nassau groupers grumpily defend their home.

[ Thrumming continues ] The calls travel long distances.

[ Thrumming continues ] Listening is key to survival.

SCHARER: Baby fish need to hear where they're going and they use the sound to guide their swimming.

This helps them then grow up on these reefs.

As they grow and they need to find where to go to spawn, they can rely on sound to find those pathways to meet up at the spawning aggregation site when it's time.

I heard something there different.

Did you hear it, too?

Yeah?

PETERSEN: Yeah, it's different.

SCHARER: Okay.

There's something in here.

NARRATOR: This rich soundscape helps make the case that the Cayman Crown must be protected.

And in 2020, science wins a big victory.

The countries sharing the reef grant the Crown conservation status.

McFIELD: We've protected this reef now.

In Guatemala, it's fully protected.

In Belize, it's highly protected.

They've stepped up to the plate and protected it.

So, you know, that was our ultimate goal, is, "Hey, we need to protect this," and now it's happened.

♪♪ This helps, but I think we need to now really work on management and enforcement and do all that we can to ensure that this reef does have all the elements that will help it be resilient and that fish are on the reef and not being taken.

♪♪ NARRATOR: And, indeed, the conservation win appears short-lived.

♪♪ Michelle is already finding evidence that the Crown's future is in jeopardy.

It's spawning time, the same season Will heard groupers two years ago.

♪♪ Now, Michel detects a shift.

♪♪ SCHARER: We did not hear as many as we would expect.

♪♪ So I think there may be something going on.

NARRATOR: In fact, the groupers seem to have almost disappeared from the Crown.

♪♪ ♪♪ The news stuns the team.

HEYMAN: Fishers have realized that this is a gold mine.

You know, I'm not an emotional guy, but how do you deal with this?

McFIELD: How do we stop it?

How do we stop the degradation?

NARRATOR: Fishers must have been targeting the Crown.

But it's not the locals.

Kenny has been working with him.

He's confident they are respecting a fishing ban the government's put in place.

[ Indistinct conversations ] The team suspects the raiders are from outside Belize and Guatemala.

MARTIN: We've been seeing nets, fishing line, even starting and batting into the reef.

I've been seeing anchors that are used to moor their boats when they go fishing.

We've been seeing a lot of broken corals, ripped-out sponges, and stuff like that.

So we know that fishermen are targeting it a lot for commercial species.

NARRATOR: A sponge is sliced cleanly by a fishing line.

Kenny and Melanie stitched the pieces together.

♪♪ But the team can only do so much.

♪♪ MARTIN: I would like to see more protection.

I would like to see patrol boats out there.

I would like to see Belize, Guatemala, and Honduras working together to protect these resources that is very vital to fishermen's livelihood and also for the local people.

NARRATOR: The current protections, incomplete as they are, are placing the guardians of the reef at risk.

HEYMAN: They're protecting globally important, valuable resources on the front line at night, you know, with people coming out there with guns.

It's dangerous.

This is protecting gold.

♪♪ [ Generator whirring ] PETERSEN: It's really alarming.

You know, we need to think about what to -- what to do.

♪♪ We need to bring back our fish.

♪♪ NARRATOR: As the team calls for enforcement, Michelle ponders whether groupers can be lured back to the Crown.

She takes inspiration from the marine sanctuary of Glover's Reef, five hours north by boat.

SCHARER: For a few years, I've been collaborating with Myles Phillips, recording the sounds of the groupers at the northeast point of Glover's, specifically listening to the sounds produced by the Nassau grouper to be able to detect when they are supposed to spawn.

NARRATOR: Those recordings suggest that the groupers are starting to gather.

SCHARER: These fish have to migrate quite a way from different distances.

They need to know they're on the right track.

So communication between them as they form little groups and move towards the main aggregation site is really important for them.

NARRATOR: Nassau groupers are solitary most of the year.

Reproduction brings them together.

Some will travel hundreds of miles.

♪♪ ♪♪ They arrive in December and January, when Caribbean waters are coolest.

SCHARER: They start communicating to each other.

They have different signals, different languages or different words or vowels for the different behaviors that they conduct only when they aggregate to spawn.

NARRATOR: They also communicate with stylized movements and shifting patterns on their skin.

HEYMAN: They look cool already.

They got their stripes and everything.

But when it's getting close to spawning, they're, like, trying on different outfits.

Some turn white, some turn black.

NARRATOR: More groupers arrive.

The chorus swells in volume.

[ Loud thrumming ] ♪♪ Some wait on the sea floor, conserving their energy.

SCHARER: They need to synchronize to 1 or 2 nights.

Everybody has to have their eggs and their sperm ready for that moment.

They are communicating, "Are we ready to spawn?

Do you think we can spawn now?"

[ Thrumming continues ] NARRATOR: As their numbers grow, they rise from the sea floor each evening, as if rehearsing.

[ Thrumming continues ] As the full moon approaches, the performance has become more sophisticated.

HEYMAN: Where the real fun is, is in the courtship prior to spawning.

Each has their own suite of behaviors and sounds that all come together into this courtship dance.

[ Grouper thrums ] NARRATOR: If they are ready, they change into matching costumes.

HEYMAN: But as you get closer to spawning, they all choose this tuxedo, it looks like to me -- black, black, black, with a bar over the eye, and a white belly.

We call it bicolor phase.

[ Grouper thrumming ] It's like a vaudeville act.

Males fighting over females, making sounds at each other.

[ Thrumming continues ] NARRATOR: Males wrestle around the females.

The noise builds.

[ Grouper thrumming ] But night after night, they hold back.

[ Grouper thrumming ] Then, one night, the temperature is right.

The moonlight is right.

Most importantly, there's a critical mass of fish.

The chorus reaches a crescendo.

[ Grouper thrumming loudly ] Then, silence.

Spawning has begun.

♪♪ ♪♪ HEYMAN: These are the volcanoes of reproduction for the entire region.

All of these fish coming all together at these promontories, currents that are upwelling and shooting by there and creating and captured in gyres, some shooting way long distances to seed other places.

That is the large-scale connectivity throughout this entire wider Caribbean basin.

♪♪ ♪♪ NARRATOR: The mating spectacle at Glover's most likely used to take place at the Crown.

Could it happen here again with a helping hand?

Michelle hatches a plan to use sound recordings from Glover's to attract groupers to the Crown.

SCHARER: If there are fish nearby, whatever's left, can we actually make them recover that spawning aggregation that once was at the jewel?

HEYMAN: I'm curious.

SCHARER: How about we put out loudspeakers and play these sounds and see if we can attract whatever's left of what used to be the aggregation at the jewel?

HEYMAN: Okay.

SCHARER: This would be the first try to attract fish that maybe are still there, but they've lost their cues because there's only a few fish left.

So if we play back these sounds and they recognize them, maybe they'll come back to the aggregation site.

NARRATOR: It's not such a wild idea.

In Australia, sound recordings were used to lure baby fish to settle on a recovering reef.

Michelle and Kenny test an underwater speaker at the Crown.

The first step is to gauge how far the sound travels.

[ Grouper thrumming plays ] If the experiment works, it could jumpstart the recovery of fisheries at the Crown.

[ Grouper thrumming plays ] But recovery will also depend on staunch enforcement of the fishing ban.

[ Grouper thrumming plays ] Groupers can recover.

Strict protection of two aggregations in the Cayman Islands tripled their numbers in under a decade.

The team listens to the grouper calls coming from the speakers below.

They anticipate what might happen.

[ Thrumming ] MARTIN: Imagine hundreds of them coming up and "Kkkkkk," just making that thing happen.

HEYMAN: Oh, listen to that.

SCHARER: I guess it takes one first one to go off, and then everybody follows in queue.

PHILLIPS: They see one person go to the dance, where they're like, [snaps] "That's it."

SCHARER: Mm-hmm.

HEYMAN: Yeah.

[ Thrumming continues ] ♪♪ NARRATOR: When the Crown was discovered, it sparked a sense of hope that the Caribbean's embattled coral reefs may not be doomed.

And it has brought once unlikely allies together.

MARTIN: It's gonna be a huge change to have fishermen thinking on a positive note to protect these resources so they could replenish the areas.

♪♪ NARRATOR: The team's success in exploring and protecting this natural wonder has given them encouragement to carry on.

McFIELD: We all get refreshed, and our enthusiasm and our hope for what we're doing, that it -- we can succeed and we will succeed if we keep pushing.

So that's what we do.

NARRATOR: The Crown itself has also demonstrated a remarkable resilience.

And the fight for its future has only just begun.

HEYMAN: I've seen places recover.

If we're -- If we're gentle, if we're good, if we realize the value, these places are resilient.

These places can come back.

And that's why I maintain hope.

♪♪ ♪♪ PHILLIPS: It's not even a little ray of hope.

This place is huge.

It's like -- It's like seeing the sun spilling out of the clouds.

♪♪ ♪♪ ♪♪ ♪♪ ♪♪ ♪♪ ♪♪ To learn more about what you've seen on this "Nature" program, visit pbs.org.

♪♪