ANTHONY MENDEZ: MEXICO.

IN THE FAR SOUTHEAST, THE YUCATAN PENINSULA, A LAND ONCE RULED BY MIGHTY MAYA KINGDOMS... [BIRDS CHIRPING] SWATHED IN A GREAT FOREST FULL OF NATURAL WONDERS.

[MONKEY HOWLS] [ALLIGATOR GROWLS] [THUNDER] JOURNEY THROUGH THE SEASONS... AND DISCOVER A HIDDEN UNDERWORLD... A VAST FLOODED LABYRINTH THAT FEW HAVE EVER SEEN.

THESE SECRET RIVERS SHAPE THE LIVES OF ALL THAT LIVE HERE.

MEXICO, OUR NEIGHBOR TO THE SOUTH, IS A COUNTRY RICH IN CULTURE, ABLAZE WITH COLOR, A FESTIVAL OF LIFE.

IN MEXICO'S FAR SOUTH, THERE'S A FOREST FULL OF SECRETS.

IT'S MARCH, THE PEAK OF THE DRY SEASON.

THIS POOL IS FAST DISAPPEARING, AND IT DRAWS ALL THE ANIMALS FROM THE FOREST...

EVEN THE ELUSIVE JAGUAR.

FOR A YOUNG MORELET'S CROCODILE, THE POOL IS ALSO A HUNTING GROUND.

THERE'S PLENTY OF POTENTIAL PREY.

TROUBLE IS, MOST ARE FAR TOO BIG.

HE'S ONLY 3-FEET LONG.

FINALLY, A REALISTIC TARGET.

[BIRDS COOING] EVEN AT HIS SIZE, SNEAKING UP IN SUCH SHALLOW WATER ISN'T EASY.

PERHAPS AN AMBUSH WILL WORK.

ALMOST.

[BIRDS COOING] SUCCESS AT LAST.

BUT HE'S NOT THE ONLY CROCODILE IN THIS POOL.

HIS TASTE OF VICTORY IS SHORT LIVED.

THE REASON WATER IS SO SCARCE HERE IS BECAUSE OF THE YUCATAN'S UNIQUE GEOLOGY.

THE ENTIRE PENINSULA IS AN ENORMOUS SLAB OF LIMESTONE.

LIMESTONE IS INCREDIBLY POROUS.

ANY SURFACE WATER SWIFTLY DRAINS AWAY UNDERGROUND.

AND BECAUSE OF THIS, THE YUCATAN HAS NO MAJOR RIVERS.

DESPITE THE FOREST'S LUSH APPEARANCE, VIRTUALLY NO RAIN FALLS FOR 7 MONTHS OF THE YEAR.

BY APRIL, MUCH OF THE FOREST IS BARREN.

FOR A MOTHER SPIDER MONKEY, THIS LONG, DRY SEASON CAN BE CHALLENGING.

SHE NEEDS TO PRODUCE ENOUGH MILK TO FEED HER BABY, AND THAT REQUIRES FINDING PLENTY OF FRESH FRUIT.

SOMETIMES THAT MEANS TRAVELING OVER TWO MILES EACH DAY.

BUT WITH LONG, SLENDER FINGERS AND NO THUMBS TO GET IN THE WAY, THIS IS CHILD'S PLAY FOR AN ANIMAL DESIGNED FOR LIFE IN THE TREETOPS.

SHE'S TAKING HER YOUNGSTER TO A SPECIAL PLACE WHERE SHE KNOWS THERE'S FRUIT ALL YEAR ROUND...

THE ANCIENT OF CITY OF CALAKMUL, ONCE ONE OF THE MOST POWERFUL IN THE MAYA CIVILIZATION.

IN THE SEVENTH CENTURY, THIS CITY WAS HOME TO 50,000 PEOPLE.

THE PEOPLE OF CALAKMUL CULTIVATED TREES THAT WOULD FRUIT THROUGHOUT THE YEAR.

CENTURIES LATER, THESE ANCIENT GARDENS LIVE ON.

[MONKEYS CHATTERING] THERE'S MORE FRUIT HERE THAN ANYWHERE ELSE IN THE FOREST, EVEN AT THE HEIGHT OF THE DRY SEASON.

OVER COUNTLESS GENERATIONS, THE MONKEYS HAVE PASSED THIS KNOWLEDGE DOWN TO THEIR YOUNGSTERS.

[MONKEYS VOCALIZING] AT ITS PEAK IN THE EIGHTH CENTURY, THE MAYA CIVILIZATION GREW TO ALMOST 13 MILLION PEOPLE.

THERE'S WAS A HIGHLY SOPHISTICATED CULTURE, ADVANCED IN MATHEMATICS, LANGUAGE, AND ASTRONOMY.

THE HUGE LIMESTONE TEMPLES BUILT TO WORSHIP THE GODS ARE A LASTING TESTAMENT TO GREAT FEATS OF ENGINEERING.

ALMOST EVERY OTHER ANCIENT CIVILIZATION IN HISTORY HAS BEEN BUILT BESIDE A MAJOR RIVER.

SO HOW DID THE MAYA MANAGE WITHOUT ONE?

THERE ARE ALMOST 1 1/2 MILLION DIRECT DESCENDANTS OF THE ANCIENT MAYA LIVING IN MEXICO TODAY.

DON ROQUE AND HIS WIFE DONA SU LIVE ON AN ISOLATED FARM IN THE HEART OF THE YUCATAN'S FOREST.

[SHEEP BLEATS] MENDEZ: THIS FARM HAS BEEN IN DON ROQUE'S FAMILY FOR GENERATIONS.

WITH HIS CHILDREN GROWN UP AND LIVING IN A VILLAGE NEAR BY, NOWADAYS IT'S JUST HIM AND DONA SU.

LIKE MANY MODERN DAY MAYA, THEY LIVE OFF THE LAND IN A WAY LITTLE CHANGED SINCE THE TIME OF THEIR ANCESTORS.

THEY'VE CLEARED A SMALL PLOT IN THE FOREST TO RAISE LIVESTOCK AND GROW THEIR OWN FOOD.

BUT THERE'S MORE TO THEIR FARM THAN MEETS THE EYE.

A HUGE NATURAL WELL, KNOWN IN THE YUCATAN AS A CENOTE.

A CENOTE IS A COLLAPSED CAVE.

THOUSANDS OF YEARS OF RAINWATER HAS ERODED AREAS OF SOFT LIMESTONE.

EVENTUALLY, THE SURFACE WEAKENED AND CAVED IN.

THIS CENOTE PROVIDES DON ROQUE WITH WATER ALL YEAR ROUND.

AND IT IS FAR FROM UNIQUE.

IT'S ONE OF SEVERAL THOUSAND CENOTES SCATTERED ACROSS THE PENINSULA.

"CENOTE" COMES FROM THE MAYA WORD "TS'ONOT," MEANING HOLE WITH WATER.

IT WAS THESE MYSTERIOUS POOLS THAT ALLOWED THE MAYA CIVILIZATION TO FLOURISH WITHOUT A MAJOR RIVER, AND THE GREATEST ANCIENT CITIES WERE FOUNDED BESIDE THEM.

EACH CENOTE IS AN OASIS, SUSTAINING LIFE THAT OTHERWISE COULDN'T EXIST HERE.

THE SURROUNDING FOREST REMAINS LUSH, EVEN AT THE HEIGHT OF THE DRY SEASON.

FILTERED THROUGH THE LIMESTONE, THE WATER IS FULL OF MINERALS AND SUPPORTS A UNIQUE COMMUNITY OF PLANETS AND ANIMALS.

SOME OF THE FISH IN THE YUCATAN CENOTES ARE FOUND NOWHERE ELSE ON EARTH.

WITHIN THE DENSE FOREST, A CENOTE CAN REMAIN HIDDEN EVEN WHEN CLOSE BY.

[ANIMAL SCREECHES] BUT THE MAYA HAVE AN ALMOST SURE-FIRE WAY OF FINDING THEM... [ANIMAL SCREECHES] THE DISTINCTIVE CALL OF THE TURQUOISE-BROWED MOTMOT.

[BIRD CALLS] VIRTUALLY EVERY CENOTE HAS ITS OWN COLONY.

ELSEWHERE, MOTMOTS PREFER TO NEST ALONE, DIGGING THEIR BURROWS ALONG RIVERBANKS.

[BIRD CALLS] HERE IN THE YUCATAN, THEY'RE FORCED TOGETHER, SHARING LIMITED SPACE ON THE CENOTE'S SOFT LIMESTONE WALLS.

BEFORE A MALE CAN START LOOKING FOR A MATE, HE NEEDS TO SECURE A NEST SITE, BUT THIS CENOTE'S ALREADY CROWDED.

17 OTHER PAIRS ARE NESTING HERE.

[BIRD CALLS] IF HE'S TO FIT IN, THIS NEW ARRIVAL MUST USE THE RIGHT BODY LANGUAGE.

[CALLING] [CALLING] A RAISED TURQUOISE BROW IS INTENDED TO INTIMIDATE RIVALS.

[CALLING] [CALLING] THE PENDULOUS TICK-TOCK OF THEIR TAILS HAS EARNED MOTMOTS THE NICKNAME CLOCK BIRDS.

THIS MALE MUST PERSEVERE IF HE WANTS TO BREED THIS YEAR.

[CALLING] AT LAST, HE CLAIMS HIS OWN LITTLE LEDGE OF LIMESTONE.

WITH THE GIFT OF FOOD, HE CAN GET DOWN TO THE BUSINESS OF ATTRACTING A MATE.

THERE ARE MANY CENOTES IN THE YUCATAN, MOST CREATED BY WATER EROSION, BUT SOME WERE FORGED BY A SINGLE EVENT THAT LEFT A LASTING IMPRESSION ON THE WHOLE PENINSULA AND CHANGED THE HISTORY OF LIFE ON EARTH.

AROUND 65 MILLION YEARS AGO, A HUGE METEORITE COLLIDED WITH WHAT IS NOW THE YUCATAN'S NORTH COAST.

ITS IMPACT ON THE GLOBAL CLIMATE WAS CATASTROPHIC.

AND IT'S BELIEVED TO HAVE WIPED OUT THE DINOSAURS.

IN AN INSTANT, THE METEOR FRACTURED THE LIMESTONE PLATFORM, CREATING WHAT'S KNOWN AS THE RING OF CENOTES.

SATELLITES HAVE REVEALED OVER 200 CLUSTERED AROUND ITS CRATER.

SATELLITE TECHNOLOGY IS ALSO HELPING MODERN-DAY EXPLORERS TO THE YUCATAN, AND NEW CENOTES ARE STILL BEING DISCOVERED TODAY.

IT SHOULD BE ABOUT 50 METERS THAT WAY.

YOU SEE ANYTHING?

MENDEZ: SAM MEACHUM AND FRED DEVOS ARE PART OF AN INTERNATIONAL COMMUNITY OF CAVE DIVERS THAT HAS BEEN EXPLORING THE YUCATAN SINCE THE 1980S.

MAN: I STARTED OUT FOR THE THRILL OF BEING ABLE TO EXPLORE SOMETHING.

AND FROM A YOUNG AGE, I JUST WAS VIEWED WITH THE DESIRE TO GO OUT AND EXPLORE THIS WORLD.

MENDEZ: WITH INFORMATION FROM SATELLITES LOADED ONTO HIS GPS, SAM HAS IDENTIFIED AN AREA OF FOREST WHERE THE VEGETATION CONTAINS MORE MOISTURE.

THIS MEANS IT'S LIKELY THERE'S A CENOTE THERE, EVEN IF THERE'S NO WATER VISIBLE FROM ABOVE.

UH-HUH.

WOW, LOOK AT THIS, SAM.

SAM: WOW.

SAM: THIS IS DEFINITELY MY FAVORITE BIT.

THE MOMENT OF DISCOVERY, RIGHT?

YOU CAN'T BEAT IT.

MENDEZ: EACH NEW CENOTE ADDS MORE INFORMATION TO THE ON-GOING STUDY.

IN THE LAST 30 YEARS, EXPLORERS IN THE YUCATAN HAVE REVEALED SOMETHING EXTRAORDINARY.

THE CENOTES ARE NOT ISOLATED WELLS.

ALL ARE CONNECTED BY AN INTRICATE NETWORK OF CAVES AND PASSAGEWAYS SPREADING LIKE A SPIDER'S WEB ACROSS THE PENINSULA.

DIVERS HAVE DISCOVERED OVER 350 CAVES AND NEARLY A THOUSAND MILES OF FLOODED TUNNELS, INCLUDING THE TWO LONGEST UNDERWATER CAVE SYSTEMS ON THE PLANET.

MANY OF THESE CAVERNS HAVE SEEN FEWER VISITORS THAN THE MOON, BUT THE GREAT MAJORITY REMAIN UNCHARTED.

FOR THE ANCIENT MAYA, THE LABYRINTH OF CAVES WAS MOSTLY OUT OF REACH, BUT NOT BEYOND IMAGINATION.

THIS SECRET UNDERWORLD WAS HELD SACRED, HOME TO THE 12 GODS OF XIBALBA.

IT WAS A PLACE BOTH FEARED AND REVERED.

BUT THE MAYA RECOGNIZED A LINK TO THIS UNKNOWN WORLD-- TREES.

THEIR ROOTS CAN PUSH THROUGH FISSURES AND FRACTURES IN THE LIMESTONE.

[BIRDS CHIRPING] BY DRAWING THE WATER FROM BELOW, THE TREES HELP LIFE ABOVE GROUND TO FLOURISH.

NO TREE IS BETTER EQUIPPED TO DO THIS THAN THE STRANGLER FIG.

ITS ROOTS CAN GROW 100 FEET LONG.

DON ROQUE USES THE FIG'S ROOTS TO ENTER THE UNDERWORLD, NAVIGATING HIS WAY DOWN STEPS CARVED BY HIS GRANDFATHER OVER HALF A CENTURY AGO.

MENDEZ: AT THE BOTTOM OF THE CENOTE, DON ROQUE HAS A SECRET GARDEN.

HERE, HE CULTIVATES A PLANT THAT WOULDN'T SURVIVE ELSEWHERE ON THIS FARM-- COFFEE.

UP ABOVE, IT'S TOO HOT AND DRY FOR COFFEE TO GROW.

DOWN HERE, IT'S TWO DEGREES COOLER AND A LOT MORE HUMID.

DON ROQUE'S CENOTE IS ALSO A HAVEN FOR INSECTS, FOOD FOR A COLONY OF RESIDENT CAVE SWALLOWS.

[BIRDS CHIRPING] BY MAY, THE SWALLOWS ARE INTENT ON ONE THING-- BREEDING.

THE CENOTE PROVIDES EVERYTHING THEY NEED.

DOWN HERE, THEY HAVE A READY SUPPLY OF SOFT MUD, WHICH THEY USE TO CRAFT THEIR NESTS...

TOGETHER WITH FIBERS PLUCKED FROM THE HANGING FIG ROOTS.

IF ALL GOES WELL, IN AROUND A MONTH'S TIME, THEY'LL EACH HAVE A NEW BROOD OF UP TO 5 CHICKS.

DON ROQUE'S CENOTE HAS ITS OWN COMMUNITY OF LIFE THANKS TO THE CONSTANT PRESENCE OF WATER.

BUT THE REST OF HIS FARM REQUIRES SOMETHING MORE.

HE PLANTS HIS LAND WITH A MIXTURE OF CORN, BEANS, AND SQUASH.

BUT HE CAN'T DRAW ENOUGH WATER FROM HIS CENOTE TO IRRIGATE AN ENTIRE FIELD.

FOR HIS CROPS TO GROW, DON ROQUE NEEDS IT TO RAIN.

BY LATE MAY, CHANGE IS IN THE AIR.

THE YUCATAN RELIES ON WEATHER SYSTEMS THAT DEVELOP THOUSANDS OF MILES AWAY.

GATHERING MORE AND MORE WATER AS THEY CROSS THE ATLANTIC, THEY DELIVER THE SEASONAL RAINS.

THE ANCIENT MAYA BELIEVED THE RAIN WAS A GIFT FROM THE GODS.

FOR DON ROQUE, THE OLD GODS ARE VERY MUCH ALIVE.

[THUNDER] THE MOST IMPORTANT OF ALL IS CHAAC, THE LIFE-GIVING GOD OF RAIN.

[THUNDER] [THUNDER] IT IS CHAAC WHO DETERMINES EACH THUNDERCLOUD, FINALLY LETTING THE RAIN FALL.

1,200 YEARS AGO, THE MAYA CIVILIZATION SUFFERED A DRAMATIC DECLINE.

CHAAC WITHHELD THE SEASONAL RAINS.

THE YUCATAN WAS GRIPPED BY A SERIES OF DEVASTATING DROUGHTS.

OVER 80% OF THE POPULATION VANISHED.

ONE BY ONE, THE BUSTLING CITIES WERE ABANDONED AND RECLAIMED BY THE FOREST.

[MONKEYS HOWLING] THIS YEAR THE GODS HAVE BEEN KIND.

BY JULY, IT'S RAINING ALMOST EVERY DAY.

[MONKEYS HOWLING] THIS BRINGS NEW GROWTH TO THE FOREST.

GOOD NEWS FOR THE YUCATAN'S NOISIEST RESIDENTS... [HOWL] [MONKEYS HOWLING] HOWLER MONKEYS.

THEIR CALLS CARRY FOR OVER A MILE.

[MONKEYS HOWLING] THEY HOWL TO LET NEIGHBORING GROUPS KNOW THEIR WHEREABOUTS.

THIS PATCH OF THE FOREST IS WORTH PROTECTING.

THE TROOP HAS FOUND A TREE WITH YOUNG, SUCCULENT LEAVES-- A PARTICULAR FAVORITE.

A FULLY GROWN HOWLER MONKEY CAN EAT OVER TWO POUNDS IN ONE SITTING.

BUT A DIET OF LEAVES TAKES TIME TO DIGEST...

SO HOWLER MONKEYS SPEND UP TO 80% OF THEIR TIME RESTING, MAKING THEM AMONGST THE MOST PEACEFUL OF ALL MONKEYS DESPITE THEIR FEARSOME CALLS.

BABY HOWLERS FEED ON THEIR MOTHER'S MILK UNTIL THEY'RE ALMOST A YEAR OLD.

THIS IS MUCH EASIER TO DIGEST, LEAVING LOTS OF TIME TO PLAY.

WITH HIS MOTHER TRYING TO SLEEP OFF HER LUNCH, NAPTIME CAN BE A BIT DULL FOR A BABY.

[THUNDER] ALL LIFE IN THE YUCATAN BENEFITS FROM THE RAIN.

TREES BURST INTO FRUIT.

NEW PLANT GROWTH PROVIDES FOOD FOR AN EXPLOSION OF INSECT LIFE.

THERE ARE OVER 3,000 DIFFERENT SPECIES OF INSECT ON THE PENINSULA.

THE INSECTS BECOME FOOD FOR OTHERS.

ALMOST ALL THE BIRDS IN THE FOREST SCHEDULE THEIR NESTING TO COINCIDE WITH THIS SEASONAL BOUNTY.

THE REGULAR RAIN IS ALSO GOOD NEWS FOR DON ROQUE.

IN JUST 6 WEEKS, HIS CORN HAS SHOT UP.

MENDEZ: BUT THE EXPLOSION OF PLANT-EATING INSECTS COULD CAUSE DON ROQUE PROBLEMS.

FOR HIS CROPS TO THRIVE, HE NEEDS THE HELP OF A NATURAL ALLY.

IN THE HEART OF THE YUCATAN, THERE'S A CAVE THE LOCALS CALL EL VOLCAN DE LOS MURCIELAGOS-- THE BAT VOLCANO.

EVERY DAY, AN HOUR BEFORE DUSK, IT ERUPTS.

[BATS SQUEAKING] AS MANY AS 3 MILLION BATS EXIT THE CAVE EACH EVENING.

THE ANCIENT MAYA ASSOCIATED BATS WITH DEATH.

THIS WAS A SWARM STRAIGHT FROM THE UNDERWORLD.

BUT BATS ARE VITAL TO LIFE IN THE YUCATAN, KEEPING ITS INSECT POPULATION IN CHECK.

EACH CAN EAT ITS OWN BODY WEIGHT IN INSECTS EACH NIGHT.

WHILE THE BATS ARE FOCUSED ON THE INSECTS, OTHERS ARE WATCHING THEM.

GREY HAWKS... AND BAT FALCONS.

THESE BIRDS ARE SKILLED AERIAL PREDATORS.

THEIR SHARP TALONS ARE PERFECTLY DESIGNED TO SNATCH BATS FROM THE AIR.

[BATS SQUEAKING] THERE ARE OTHER MORE SURPRISING HUNTERS THAT THRIVE IN THESE LOWLAND FORESTS.

BROWN JAYS.

THEY USUALLY EAT INSECTS AND FRUITS, BUT THESE BROWN JAYS HAVE DEVELOPED A TASTE FOR BATS.

THE JAYS DON'T USE TALONS TO CATCH THEIR PREY.

THEY USE THEIR BEAKS.

THE LOSS OF JUST A FEW BATS EACH EVENING HARDLY MAKES A DENT ON THIS 3 MILLION STRONG SWARM.

TONIGHT, THESE BATS WILL CONSUME AROUND 30 TONS OF INSECTS, A VITAL SERVICE THAT KEEPS THE ENTIRE FOREST HEALTHY.

[THUNDER] [SHEEP BLEATS] IT'S LATE SEPTEMBER, AND IT HAS RAINED VIRTUALLY EVERY DAY FOR THE LAST 3 MONTHS.

DON ROQUE'S CORN IS NOW RIPE AND READY FOR PICKING.

MENDEZ: IN A TRADITION DATING BACK TO THE TIME OF THE ANCIENT MAYA, THE FIRST CORN OF THE HARVEST SERVES A SPECIAL PURPOSE.

DONA SU CRUSHES IT TO MAKE A SACRED DRINK CALLED ATOLE.

THIS IS OFFERED TO THE GODS IN A CEREMONY OF THANKSGIVING.

[SPEAKING NATIVE LANGUAGE] MENDEZ: FROM JUNE TO NOVEMBER, THE YUCATAN RECEIVES OVER TWO FEET OF RAIN.

ACROSS THE PENINSULA, THAT'S OVER 36 TRILLION GALLONS.

THE CENOTES ARE REFILLED.

BUT MOST OF THIS WATER SEEPS THROUGH THE LIMESTONE INTO THE UNDERWORLD.

ALL WHO LIVE IN THE YUCATAN RELY ON THIS UNIQUE WATER RESOURCE TO SURVIVE.

AND IN THE LAST 50 YEARS, THE PENINSULA'S HUMAN POPULATION HAS SKYROCKETED, ESPECIALLY ALONG THE CARIBBEAN COAST.

THIS 100-MILE STRETCH OF THE YUCATAN IS ONE OF THE FASTEST-GROWING TOURIST DESTINATIONS IN THE WORLD.

IT'S CREATING NEW JOBS, AN IMPORTANT SOURCE OF INCOME FOR THE WHOLE OF MEXICO.

BUT SUCH RAPID DEVELOPMENT STANDS ON FRAGILE FOUNDATIONS.

THE GROWING POPULATION IS PUTTING PRESSURE ON THE PENINSULA'S FRESHWATER SYSTEM.

SO THE IDEA IS WE'VE GOT A... [INDISTINCT] SHOULD BE PRETTY OBVIOUS WHEN WE GET CLOSE TO IT.

MENDEZ: THE GREAT MAJORITY OF THE YUCATAN'S UNDERWORLD REMAINS UNCHARTED.

SAM: THIS IS EXPLORATION WITH REAL PURPOSE.

IT IS REALLY CRITICAL IN HELPING OUR UNDERSTANDING OF THE SUPPLY OF FRESH WATER FOR THIS ENTIRE REGION.

MENDEZ: BY REVEALING THE WHEREABOUTS OF THE WATER UNDERGROUND, CAVE DIVERS LIKE SAM HOPE TO HELP PROTECT THIS FRAGILE ECOSYSTEM.

MAPPING THE UNDERWORLD IS A PAINSTAKING PROCESS... WITH EVERY MEASUREMENT TAKEN BY HAND.

BUT BY DOING ALL THIS CAREFUL WORK, THE CAVE DIVERS HAVE REVEALED SOMETHING REMARKABLE.

THE WATER IN THESE CAVES IS NOT SIMPLY LOCKED WITHIN THE LIMESTONE, IT FLOWS.

THESE ARE THE YUCATAN'S MISSING RIVERS.

LIKE ANY RIVER, THE WATER UNDERGROUND IS COMPELLED TOWARDS THE COAST.

HERE, THE FRESH WATER MEETS SALT WATER BROUGHT BY THE INCOMING TIDE.

THIS BORDER IS CALLED THE HALOCLINE.

CONTAINING MORE DISSOLVED PARTICLES, THE SALT WATER IS HEAVIER AND LIES BELOW THE FRESH WATER, GIVING THE ILLUSION OF A SURFACE.

BUT AFTER MONTHS OF HEAVY RAIN, THE FLOW OF FRESH WATER PREVAILS.

AT THE COAST, IT LEAVES THE UNDERWORLD BEHIND.

MANGROVES FORESTS FLOURISH IN THE MIXTURE OF SALT AND FRESH WATER.

THEIR ROOTS PROVIDE IDEAL NURSERIES FOR SCHOOLS OF FISH.

AND GENTLE GIANTS ARE DRAWN TO THE YUCATAN'S SHORES.

MANATEES COME TO DRINK FROM THE OUTPOURING OF FRESH WATER.

AS THE FRESH WATER FLOWS THROUGH THE MANGROVES, THE TREES RELEASE TANNINS, STAINING IT BROWN.

EVENTUALLY THE FRESH WATER REACHES ITS FINAL DESTINATION.

IT POURS OUT INTO THE GULF OF MEXICO IN HUGE VOLUMES.

VAST COASTAL LAGOONS, REPLENISHED BY FRESH WATER, MAKE THE IDEAL HOME FOR A COLONY OF THE YUCATAN'S MOST COLORFUL RESIDENTS... [FLAMINGOS SQUAWKING] CARIBBEAN FLAMINGOS.

IT'S BEEN A GOOD YEAR FOR THIS COLONY, WITH NEARLY 15,000 NEW ADDITIONS.

FINDING YOUR CHICK IN SUCH A BIG CROWD ISN'T EASY.

[FLAMINGOS SQUAWK] BUT FLAMINGOS HAVE INCREDIBLY GOOD HEARING AND HER YOUNGSTER HAS A UNIQUE CALL.

JUST 6 WEEKS AGO, THE CHICK WAS NOT MUCH BIGGER THAN HIS MOTHER'S BEAK.

THESE NUTRIENT-RICH LAGOONS ARE FULL OF BRINE SHRIMP.

IT'S THE BETA-CAROTENE IN THESE TINY CRUSTACEANS THAT GIVES THE FLAMINGOS THEIR DISTINCTIVE COLOR.

BUT IT WILL BE ANOTHER TWO YEARS BEFORE THE CHICKS START TO TURN PINK.

HERE ON THE COAST, AS ELSEWHERE ON THE PENINSULA, ALL LIFE OWES ITS SURVIVAL TO THE YUCATAN'S UNDERGROUND WATER WORLD.

WITHOUT ITS GREAT HIDDEN RIVERS, THIS WOULD BE A HOT, DRY, AND HOSTILE PLACE.

BORN OF A UNIQUE HISTORY, THE UNDERWORLD HIDDEN BENEATH THIS VAST FOREST GAVE RISE TO THE ANCIENT MAYA CIVILIZATION, AND IT REMAINS THE LIFEBLOOD OF THE YUCATAN.

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