ANTHONY MENDEZ: MEXICO, A LAND DOMINATED BY GREAT MOUNTAIN RANGES... STRETCHING THE LENGTH OF THE COUNTRY FOR OVER 2,000 MILES.

COME ON A JOURNEY DOWN ITS ROCKY SPINE TO DISCOVER AN INCREDIBLE DIVERSITY OF LIFE...

IN ITS MANY DIFFERENT MOUNTAIN WORLDS.

FROM THE ANCIENT MOUNTAINS OF THE NORTH... TO THE YOUNGER, MORE VOLATILE HEART... AND ON TO ITS TROPICAL CLOUD FORESTS IN THE SOUTH.

EXPLORE OUR SOUTHERN NEIGHBOR--MEXICO, A COUNTRY FULL OF WONDER.

THE BIRTHPLACE OF GREAT CIVILIZATIONS... ABLAZE WITH COLOR AND CULTURE.

A FESTIVAL OF LIFE.

[BIRDS CHIRPING] [ANIMAL CHITTERING] IN THE FAR NORTHEAST OF MEXICO IS AN ANCIENT MOUNTAIN WORLD... SERRANIAS DEL BURRO.

[BIRDS CHIRPING] IT'S HOME TO A WILD ARRAY OF CREATURES... [TURKEYS GOBBLING] [CRACK] [TURKEYS GOBBLING] AND ITS MOST SURPRISING RESIDENT IS THE BLACK BEAR.

THIS MOTHER HAS 3 YOUNG CUBS JUST 8 MONTHS OLD.

[GROWLS] [GROWLS] [GROWLS] IT'S AUTUMN, AND THE FAMILY NEEDS TO FATTEN UP BEFORE THE WINTER HIBERNATION.

LUCKILY, THE OAK TREES ARE LADEN WITH ACORNS, AN IMPORTANT SOURCE OF FAT.

THE BEARS HAVE PLENTY TO EAT, BUT THEIR CHALLENGE HERE IS FINDING ENOUGH TO DRINK.

[BIRDS TWEETING, ANIMAL CALLING] THESE LIMESTONE MOUNTAINS ACT LIKE SPONGES, SOAKING UP THE RAIN, SO THERE ARE NO PERMANENT STREAMS OR RIVERS.

[GROWLING] FORTUNATELY FOR THE BEARS, THERE IS AN ALTERNATIVE SOURCE OF WATER.

[SNUFFLING] [CATTLE LOWING] MENDEZ: SERRANIAS DEL BURRO IS PRIME CATTLE COUNTRY, AND THE RANCHERS PROVIDE YEAR-ROUND WATER FOR THEIR HERDS BY TAPPING INTO UNDERGROUND SPRINGS.

[BIRDS CHIRPING] [COW MOOS] [BIRD CALLS] [SNORTS] [BIRDS CHIRPING] [COWS MOOING] THESE WATER TANKS ARE A MAGNET FOR EVERYONE, BUT THIS MOTHER BEAR, WITH HER 3 CUBS, MUST APPROACH WITH CAUTION.

MALE BEARS SOMETIMES KILL CALVES...

SO THE RANCHERS LET THEIR COWS' HORNS GROW LONG... AS A NATURAL BEAR DETERRENT.

MENDEZ: THE FAMILY GETS THE MESSAGE, LOUD AND CLEAR.

MENDEZ: THE CATTLE HAVE HAD THEIR FILL.

NOW IT'S THE FAMILY'S CHANCE.

[BIRD SQUAWKING] BUT SOMEONE ELSE HAS GOT HIS EYE ON THE TANK.

[SNORTS] [SQUEALING] A 300-POUND MALE.

HE'S BEST AVOIDED.

[SQUEALING] THIS MALE WANTS MORE THAN A DRINK.

HE WANTS TO COOL OFF IN HIS TUB.

AT LAST.

MEXICO'S BLACK BEARS WERE ONCE ON THE BRINK OF EXTINCTION...

BUT THEY CLUNG ON IN MOUNTAIN HIDEAWAYS LIKE SERRANIAS DEL BURRO.

BY ALLOWING THE BEARS TO DRINK FROM THE WATER TANKS, AND EVEN GIVING THEM A LEG UP, THE RANCHERS HAVE REALLY HELPED THE BEARS' RECOVERY.

THE POPULATION IS FLOURISHING... MAKING THIS MOUNTAIN WORLD AN IMPORTANT STRONGHOLD FOR BEARS IN NORTH AMERICA.

MEXICO IS A VAST COUNTRY, ABOUT 2,000 MILES LONG, DOMINATED BY THE SIERRA MADRE-- A GREAT RANGE OF MOUNTAINS THAT STRETCH ALL THE WAY FROM THE COUNTRY'S NORTHERN BORDER WITH THE UNITED STATES TO ITS SOUTHERN BORDER WITH GUATEMALA.

TRAVEL DOWN THE LENGTH OF MEXICO'S ROCKY SPINE TO DISCOVER MANY MORE DIVERSE MOUNTAIN WORLDS.

THE GRANDEST OF THEM ALL IS FOUND IN THE NORTHWEST OF THE COUNTRY IN A REGION KNOWN AS COPPER CANYON, COVERING 25,000 SQUARE MILES OF WILDERNESS.

IT'S CUT BY A MAZE OF GORGES, SOME EVEN DEEPER THAN THE GRAND CANYON.

[BIRD OF PREY SCREECHES] THIS MOUNTAIN WORLD IS HOME TO A UNIQUE GROUP OF PEOPLE.

THE RARAMURI, KNOWN AS FOOT RUNNERS, HAVE LIVED HERE FOR OVER 2,000 YEARS...

SOME IN THE MOST INACCESSIBLE PLACES.

TO NAVIGATE THE CANYON'S VAST TERRAIN QUICKLY, THE RARAMURI BECAME SKILLED LONG-DISTANCE RUNNERS...

ABLE TO COVER 200 MILES WITHOUT STOPPING.

CENTURIES OF RUNNING AT HIGH ALTITUDE HAS MADE THEM UNRIVALED ENDURANCE ATHLETES.

TODAY, THE RARAMURI STILL LIVE IN HIGH, REMOTE REGIONS OF THE CANYON, AND RUNNING REMAINS AN IMPORTANT PART OF THEIR CULTURE.

SISTERS CARMEN, MEQUEJILDA, AND ELIDA ARE FROM A PROUD FAMILY OF RUNNERS.

THEIR BROTHER SANTOS IS ALREADY A LOCAL CHAMPION, WINNING MARATHONS AGAINST STRONG COMPETITION.

BUT TODAY, IT'S HIS SISTERS' TURN.

THE TRADITIONAL RUNNING SANDALS, CALLED HUARACHES, ONCE FASHIONED FROM ANIMAL HIDE, ARE NOW MADE FROM OLD CAR TIRES.

WEARING THEM FROM CHILDHOOD STRENGTHENS THE FOOT MUSCLES AND STIFFENS THE ARCHES...

PREPARING THIS NEW GENERATION OF RARAMURI FOR A LIFE ON THE MOVE.

THEY'RE HEADING TO THE NEAREST TOWN FOR A RACE.

[CHILDREN GIGGLING] [OVERLAPPING CHATTER] MENDEZ: HERE, IN THE TOWN OF POROCHI, A RACE DAY IS A CHANCE FOR DISTANT NEIGHBORS TO CATCH UP.

[CHATTER CONTINUES] THE GIRLS' RACE IS THE FIRST OF THE DAY.

[SPEAKING SPANISH] MENDEZ: THEY WILL BE COMPETING OVER 3 MILES.

UNO... DOS, TRES.

[SHOUTS FOREIGN WORD] MENDEZ: IN THIS TRADITIONAL TEAM EVENT, THE GIRLS TOSS A HOOP CALLED AN ARIWETA FOR THE FIRST LAP.

MENDEZ: ONE LAP DOWN, ONE TO GO.

THE TEAM LEADER NOW TAKES THE ARIWETA.

[CROWD SHOUTING] MENDEZ: THIS TIME, IT'S A SPRINT TO THE FINISH LINE.

SISTER ELIDA TAKES THE HOOP FOR HER TEAM.

AT LAST, THE HOMESTRETCH... [MEN SHOUTING] AND ELIDA IS FIRST TO CROSS THE FINISH LINE... [APPLAUSE] UPHOLDING HER FAMILY'S RACING HERITAGE.

THE RACES CONTINUE LONG INTO THE NIGHT.

MENDEZ: MEXICO HAS ONE OF THE LARGEST INDIGENOUS POPULATIONS IN LATIN AMERICA... MORE THAN 25 MILLION PEOPLE WITH 68 DIFFERENT LANGUAGES.

LIVING IN THE COPPER CANYON HAS MOLDED THE RARAMURI INTO THE REMARKABLE ATHLETES THEY ARE TODAY.

EACH OF MEXICO'S MOUNTAIN WORLDS SHAPES LIFE IN PROFOUNDLY DIFFERENT WAYS.

[WIND HOWLING] TRAVEL 1,000 MILES FURTHER SOUTH, DOWN THE SIERRA MADRES, AND YOU'LL DISCOVER A VERY UNUSUAL GROUP OF MOUNTAINS...

THE SIERRA GORDA, OR "FAT MOUNTAINS."

HERE, THE TEMPERATE AND TROPICAL COLLIDE, CREATING A MOSAIC OF DIFFERENT HABITATS.

[BIRDS CALLING] IN THIS ONE REGION, THERE ARE MORE THAN 2,000 DIFFERENT TYPES OF PLANTS, OVER 340 BIRD SPECIES... [HOOTS] AND 800 KINDS OF BUTTERFLY.

FAIRY-TALE OAK FORESTS ARE A SECRET GARDEN FOR MORE COLORFUL TROPICAL SPECIES...

THE MOST BEWITCHING OF ALL, ORCHIDS.

OVER MILLIONS OF YEARS, THESE FLOWERS HAVE DEVELOPED AN UNUSUAL RELATIONSHIP WITH A VERY COLORFUL CHARACTER... [BUZZING] THE ORCHID BEE.

FOR JUST A FEW MONTHS EACH YEAR, WHEN THE ORCHIDS ARE IN BLOOM, THE BEES VISIT THEM IN DROVES, NOT TO SIP THEIR NECTAR, BUT TO SWEEP UP FRAGRANT OILS FROM THEIR PETALS, USING BRUSHES ON THEIR FRONT FEET.

THEY SCRAPE THE OILS INTO A SPECIAL POUCH ON THEIR HIND LEGS.

THESE ORCHID BEES ARE MIXING THEIR OWN COLOGNE, MADE FROM UP TO 80 DIFFERENT SCENTS.

THIS COMPLEX FRAGRANCE HELPS THE MALES ATTRACT THE FEMALES.

THE MOST FRAGRANT OILS ARE PRODUCED BY ONE VERY SPECIAL KIND OF ORCHID... STANHOPEA.

THIS ORCHID BLOOMS FOR JUST ONE DAY EACH YEAR.

IN THIS CRUCIAL 24 HOURS, IT MUST SPREAD ITS POLLEN FAR AND WIDE.

[BIRDS CHIRPING] AS SOON AS IT OPENS, THE FLOWER EMITS A HYPNOTIC PERFUME THAT SMELLS LIKE FRESHLY GROUND CINNAMON... SENDING THE ORCHID BEES INTO DELIRIUM.

[BUZZING] BUT STANHOPEA'S SWEET SMELL BELIES A CUNNING MOTIVE.

ITS PETALS ARE COVERED IN SWEETENED OIL DROPLETS... AND THEIR INTRICATE SHAPE FORCES THE BEE TO WALK BACKWARDS.

THE BEE SLIPS, AND THE ORCHID STICKS A PACKET OF POLLEN ON ITS BACK, WHICH IT WILL UNWITTINGLY CARRY TO OTHER SWEET-SMELLING STANHOPEA FLOWERS.

[BIRDS CHIRPING] BY FLOWERING AND DISTRIBUTING ITS POLLEN IN JUST A SINGLE DAY, THE ORCHID'S JOB IS DONE.

NOW, AS THE SUN SETS, THE FLOWER WILTS AND DIES.

TRAVEL FURTHER SOUTH, DOWN MEXICO'S SPINE, AND ANCIENT MOUNTAINS GIVE WAY TO MUCH YOUNGER MOUNTAINS IN THE FERTILE HEART OF THE COUNTRY.

MUCH OF THIS LAND IS 6,000 FEET ABOVE SEA LEVEL.

IT'S A LANDSCAPE OF EXTENSIVE FARMLAND AND PINE-OAK FOREST.

THE SOILS HERE ARE ESPECIALLY RICH BECAUSE THIS IS A LAND OF FIERY GIANTS.

RISING MORE THAN 3 MILES ABOVE SEA LEVEL, POPOCATÉPETL IS MEXICO'S SECOND-HIGHEST PEAK AND STILL ON THE RISE.

IT'S THE COUNTRY'S MOST ACTIVE VOLCANO.

YOU NEVER KNOW WHEN IT MIGHT ERUPT NEXT.

MEXICO IS ONE OF THE MOST VOLCANICALLY ACTIVE PLACES ON EARTH BECAUSE IT LIES AT THE COLLISION POINT OF 3 TECTONIC PLATES, CONSTANTLY WRESTLING FOR POSITION.

THIS HAS PRODUCED A GREAT CHAIN OF MOUNTAINS THAT STRETCHES RIGHT ACROSS CENTRAL MEXICO KNOWN AS THE TRANSVOLCANIC BELT... WITH MORE THAN 20 ACTIVE VOLCANOES.

THEY CAN CAUSE DEATH AND DESTRUCTION, BUT THEY ARE ALSO GIVERS OF LIFE.

THEIR ASH FERTILIZES AND ENRICHES THE SOIL, MAKING THIS A HIGHLY PRODUCTIVE REGION OF MEXICO... AND THE MOST INHABITED.

THE COUNTRY'S CAPITAL, MEXICO CITY, LIES IN THE SHADOW OF VOLCANOES ON A HIGH PLATEAU IN CENTRAL MEXICO.

PEOPLE FIRST SETTLED HERE ALMOST 10,000 YEARS AGO.

IT'S A BRILLIANT SPECTACLE OF COLOR... [TRAFFIC NOISE] SOUND, CULTURE.

THIS SPRAWLING MEGA-CITY IS HOME TO OVER 20 MILLION PEOPLE, AND 80% OF ALL THE FOOD CONSUMED HERE COMES FROM A SINGLE PLACE-- THE LARGEST WHOLESALE FOOD MARKET IN THE WORLD, LA CENTRAL DE ABASTO.

[PEOPLE SHOUTING] OVER 30,000 TONS OF PRODUCE CHANGES HANDS EVERY DAY... MUCH OF IT GROWN ON MEXICO'S VOLCANIC BELT.

MAN: HEY!

THE COUNTRY'S MOST ICONIC FOODS-- CHILIS, CORN, AND AVOCADOS-- ALL ORIGINATE IN MEXICO'S HIGHLANDS.

MANY ARE EXPORTED WORLDWIDE.

AND THERE'S ONE PRODUCT THAT'S EARNED AN INFAMOUS REPUTATION.

[BELL TOLLING] TEQUILA.

[MAN SPEAKING SPANISH] 16-YEAR-OLD MARTIN IS A JIMADOR, A FARMER WHO HARVESTS BLUE AGAVE...

THE ESSENTIAL INGREDIENT IN TEQUILA.

[MARTIN SPEAKING SPANISH] AGAVE PLANTS CAN TAKE A DECADE TO MATURE BEFORE THE PLANT IS CUT DOWN.

[BARKING] AFTER SLICING AWAY THE SPIKY LEAVES, THE JIMADORES EXPOSE WHAT THEY'RE AFTER-- THE SUGAR-RICH HEART, THE PINA.

MARTIN TOILS IN THE SHADOW OF VOLCAN DE TEQUILA, WHICH LAST ERUPTED 220,000 YEARS AGO... ITS ANCIENT ASH FERTILIZING THESE RICH, RED SOILS.

HERE, IN THE WESTERN REGION OF MEXICO, THE STATE OF JALISCO PRODUCES 80% OF THE COUNTRY'S TEQUILA CRAFTED IN SPECIAL DISTILLERIES.

THE PINAS ARE FIRST ROASTED, THEN CRUSHED, SQUEEZING OUT THE SUGAR-RICH JUICES.

THESE ARE THEN FERMENTED.

FINALLY, IT'S DISTILLED...

PRODUCING THE HIGHEST-GRADE TEQUILA.

[MAN SPEAKING SPANISH] AND TO IMPROVE THE QUALITY OF THEIR CROPS, SOME FARMERS ARE TRYING SOMETHING NEW.

FOR THE FIRST TIME IN 2 CENTURIES, THEY'RE LETTING A PORTION OF THEIR AGAVE FLOWER.

THE PLANT FUNNELS ALL ITS SUGARS INTO THESE TOWERING BLOOMS, RENDERING THEM USELESS FOR TEQUILA, BUT THEY ARE NOW AN IMPORTANT DRAW FOR ANIMALS IN NEED OF A SIP OF NECTAR...

LESSER LONG-NOSED BATS.

THOUSANDS OF FEMALES MIGRATE UP MEXICO'S MOUNTAIN SPINE TO REACH BREEDING CAVES IN NORTHWEST MEXICO IN THE SOUTHWESTERN UNITED STATES.

ALONG THE WAY, THEY NEED FREQUENT SIPS OF NECTAR.

AS THEY SIP THE NECTAR, THEY HELP POLLINATE THE AGAVE.

AND BY GIVING THE PLANTS A GREATER MIX OF POLLENS, THEY'RE HELPING TO MAKE THEM STRONGER AND MORE RESILIENT TO DISEASE.

WITH THESE FARMING PRACTICES, THE AGAVE FARMERS ARE HELPING THEIR OWN FUTURE AND ENSURING THE CONTINUED LEGACY OF MEXICO'S MOST ICONIC DRINK... [GLASSES CLINK] TEQUILA.

[SCRAPING] [MEN TALKING AND LAUGHING] [CLINK] CENTRAL MEXICO'S MOUNTAINS HAVE ATTRACTED PEOPLE FOR MILLENNIA.

IMMENSE ANCIENT EMPIRES GREW IN THIS VOLCANIC HEARTLAND, AND THE REMNANTS OF SOME OF THEIR CITIES STILL STAND.

1,500 YEARS AGO, TEOTIHUACAN WAS A THRIVING MESOAMERICAN METROPOLIS, HOME TO OVER 100,000 PEOPLE.

BUT PERHAPS THE MOST FAMOUS OF ALL MEXICO'S ANCIENT EMPIRES WAS THE AZTECS.

THEY DOMINATED CENTRAL MEXICO IN THE 15TH AND 16TH CENTURIES.

AZTECS BELIEVED MOUNTAINS WERE COLUMNS CONNECTING EARTH TO THE HEAVENS AND THE UNDERWORLD.

JUST 50 MILES SOUTH OF MEXICO CITY, IS THE TEMPLE OF TEPOZTECO.

IT WAS ABANDONED LONG AGO, BUT A VERY RESOURCEFUL ANIMAL HAS SINCE MADE IT HOME... [RUNNING FOOTSTEPS] COATIS.

THEY'RE THE MOST SOCIAL OF THE RACCOON FAMILY, FORMING BANDS OF UP TO 30 STRONG.

THEY NORMALLY LIVE IN TROPICAL WOODLANDS AND OPEN FORESTS.

[SNIFFING] COATIS ARE OMNIVORES.

THEY CAN EAT PRETTY MUCH ANYTHING.

HERE AT TEPOZTECO, THERE ARE PLENTY OF OPTIONS.

[CHITTERING] [PEOPLE SPEAKING INDISTINCTLY] THIS IMPORTANT ARCHAEOLOGICAL SITE ATTRACTS MANY VISITORS, AND TOURISTS BRING OPPORTUNITIES.

THIS IS NO ORDINARY COATI CLAN.

[SNIFFING] THESE ADAPTABLE ANIMALS HAVE BECOME ARTFUL THIEVES.

BUT COATIS ARE THRIVING HERE THANKS TO TEAMWORK AND A SHAMELESS STREAK.

ALL THE VOLCANIC UPHEAVAL IN MEXICO'S CENTRAL MOUNTAINS HAS CREATED A DIVERSITY OF DIFFERENT HABITATS.

ONE OF THE MOST IMPRESSIVE IS LAKE CHAPALA, THE LARGEST FRESHWATER LAKE IN MEXICO... FED BY WATER RUNOFF FROM THE SURROUNDING MOUNTAINS.

ITS BASIN SPANS 9 MEXICAN STATES AND CAN HOLD ALMOST 2 TRILLION GALLONS OF WATER.

[ENGINE RUNNING] THE TOWN OF PETATAN ON CHAPALA'S SHORES, IS HERE BECAUSE OF THE RICH FISHING.

[MAN SPEAKING SPANISH] CHAPALA'S WATERS ALSO ATTRACT ONE OF THE LARGEST BIRDS IN MEXICO... AMERICAN WHITE PELICANS.

EVERY AUTUMN, OVER 5,000 BIRDS MAKE THE ARDUOUS JOURNEY FROM CANADA TO THESE MORE TEMPERATE WATERS.

[SPLASH] THEY ARE WELCOMED BY THE PETATAN FISHERMEN.

[MAN SPEAKING SPANISH] THE PELICANS THAT GET CLOSEST TO THE FISHING BOATS ARE IN FOR A TREAT.

[SPLASHING] [MAN SPEAKING SPANISH] SOUTH OF CENTRAL MEXICO, THE SIERRA MADRE MOUNTAIN CHAIN CONTINUES ON TOWARDS THE TROPICS.

HERE, AT ITS SOUTHERNMOST TIP, IT MEETS THE PACIFIC OCEAN.

THE MOUNTAINS CATCH THE MOISTURE FROM CLOUDS ROLLING IN OFF THE SEA.

10% OF ALL OF MEXICO'S RAIN IS CAPTURED HERE, HELPING TO CREATE A LUSH CLOUD FOREST KNOWN AS EL TRIUNFO.

THE COMBINATION OF HUMIDITY, ALTITUDE, AND WARMTH MAKES THE PERFECT CONDITIONS FOR LIFE TO FLOURISH.

[BIRDS CHIRPING] THIS SECRET MOUNTAIN WORLD IS HOME TO SOME OF THE RAREST CREATURES ON EARTH, BUT YOU HAVE TO KNOW JUST WHERE TO LOOK.

HIGH IN THE CANOPY, 100 FEET OFF THE GROUND, LIVE THE ALLIGATOR TREE LIZARD... AND THE HORNED GUAN... [SQUAWK] THE LAST SURVIVOR OF AN ANCIENT BIRD FAMILY.

IT'S BEEN EVOLVING INDEPENDENTLY FOR MORE THAN 30 MILLION YEARS.

IN THIS FOREST, THERE ARE ALSO CREATURES KNOWN FOR THEIR EXTRAVAGANT BEAUTY... LIKE LONG-TAILED MANAKINS.

MALE PAIRS PERFORM AN ELABORATE DANCE TO ENTICE FEMALES, A RITUAL THAT MAY TAKE A DECADE TO PERFECT.

[MALE MANAKIN CALLING] BUT THE MALES ARE NOT COMPETING.

THEY'RE COLLABORATING, AS MASTER AND APPRENTICE, TO GET HER ATTENTION.

ALTHOUGH ONLY ONE OF THEM MAY WIN HER HEART THIS SEASON, THE OTHER GETS VALUABLE DANCE LESSONS FOR THE FUTURE.

[MANAKINS ALTERNATE CALLING] TRAVEL DEEPER INTO THE HEART OF THIS CLOUD FOREST, AND YOU MIGHT DISCOVER THE MOST BEAUTIFUL ANIMAL OF ALL: THE SPIRIT OF EL TRIUNFO, A MYTHICAL CREATURE RARELY SEEN... SACRED TO THE ANCIENT AZTECS, WHO TREASURED ITS FEATHERS MORE THAN GOLD...

THE APTLY NAMED RESPLENDENT QUETZAL.

IT'S THE MATING SEASON, AND THIS MALE'S 3-FOOT-LONG TAIL FEATHERS HELPED HIM SUCCESSFULLY ATTRACT A FEMALE.

NOW HE HAS A YOUNG FAMILY TO FEED... AND WORKS TIRELESSLY FOR A MONTH TO SATISFY HIS BROOD'S VORACIOUS APPETITE.

HIS LUSH MEXICAN HOME IS BRIMMING WITH FRUIT AND INSECTS.

AT THE END OF THE MATING SEASON, HE WILL DROP HIS SPECTACULAR TAIL FEATHERS, BUT THE QUETZAL GROWS A NEW TRAIN EVERY SINGLE YEAR.

ONLY IN SUCH A RICH HABITAT CAN ANIMALS AFFORD TO DEVOTE SO MUCH ENERGY TO LOOKING THIS GOOD.

FROM NORTH TO SOUTH, ALL OF MEXICO'S MOUNTAIN WORLDS ARE UNIQUELY DIFFERENT, BUT ONE ABOVE ALL IS SO INVITING, IT COMPELS CREATURES TO UNDERTAKE MASSIVE FEATS OF ENDURANCE TO REACH IT.

EVERY WINTER, ONE ANIMAL TRAVELS MORE THAN 3,000 MILES TO THIS PARTICULAR FOREST IN THE CENTRAL HIGHLANDS IN THE STATE OF MICHOACAN.

THE TREES SEEM TO BE DRAPED IN LEAVES, BUT, IN FACT, THEY'RE COATED IN MILLIONS OF DELICATE CREATURES... SILENT...UNMOVING.

MONARCH BUTTERFLIES...

IN THE HUNDREDS OF MILLIONS.

THESE OYAMEL FIR FORESTS ARE THEIR PERFECT WINTER SANCTUARY.

THE LOW TEMPERATURES SLOW THE BUTTERFLIES' METABOLISM, ALLOWING THEM TO HIBERNATE FOR 5 LONG MONTHS.

MOUNTAIN MISTS PREVENT THEM FROM DRYING OUT... AND THE TOWERING FIRS HELP PROTECT THEM FROM WINTER STORMS, ENSURING THEY DON'T FREEZE.

FOR THOUSANDS OF YEARS, THESE MONARCHS HAVE HAD A SPECIAL SIGNIFICANCE FOR THE PEOPLE OF MEXICO'S MOUNTAINS.

THEY ARE BELIEVED TO BE THE SPIRITS OF THE DEAD RETURNING TO EARTH FROM THE HEAVENS, AND THEIR ARRIVAL IN WINTER COINCIDES WITH ONE OF THE MOST IMPORTANT EVENTS IN THE MEXICAN CALENDAR... [CHIMES TINKLING] [FLAMES FLICKERING] DIA DE LOS MUERTOS-- THE DAY OF THE DEAD.

[TINKLING] [SPEAKING SPANISH] IN VILLAGES ACROSS THE SIERRA MADRE, FAMILIES LIKE CATALINA'S ARE REMEMBERING THOSE WHO HAVE PASSED AWAY.

[SPEAKING SPANISH] [WOMEN SPEAKING INDISTINCTLY] CATALINA AND HER FAMILY GATHER AROUND HER HUSBAND'S GRAVE.

[CATALINA SPEAKING SPANISH] [PEOPLE SPEAKING INDISTINCTLY] CATALINA AND HER FAMILY TEND TO THEIR CANDLES UNTIL FIRST LIGHT... [STRIKES MATCH] BUT THEY'RE NOT ALONE.

TONIGHT, MILLIONS OF CANDLES ARE LIT THROUGHOUT MEXICO...

HELPING TO GUIDE THE SPIRITS OF LOVED ONES BACK HOME.

BUT THE PARTY FOR THE DEAD HAS ONLY JUST BEGUN.

[FIREWORKS WHISTLING] [WHOOPING AND CHEERING] MANY MEXICANS DO NOT BELIEVE DEATH IS AN ENDING.

THEY EMBRACE IT AS AN ESSENTIAL PART OF AN ONGOING NATURAL CYCLE.

[CHEERING CONTINUES] [BELLS JINGLING] MEXICANS HAVE BEEN CELEBRATING THE DAY OF THE DEAD FOR MORE THAN 3,000 YEARS... GENERATION AFTER GENERATION.

BUT THE SPIRITS OF THE DEAD CANNOT STAY FOR LONG.

JUST AS THE SPIRITS MUST RETURN TO THE HEAVENS, THE BUTTERFLIES WILL ALSO LEAVE.

[JINGLING] THIS IS MEXICO'S GREATEST NATURAL SPECTACLE.

UP TO A BILLION BUTTERFLIES TAKE FLIGHT.

THESE MONARCHS WILL NEVER RETURN.

THEIR SHORT TIME ON EARTH IS ALMOST OVER.

BUT THEY WILL LAY THEIR EGGS ON THEIR JOURNEY TO THE NORTHERN UNITED STATES AND SOUTHERN CANADA.

AND NEXT YEAR, THEIR DESCENDANTS WILL CONTINUE THIS ANCIENT TRADITION... BY MAKING THE SAME EPIC PILGRIMAGE ALONG MEXICO'S ROCKY SPINE BACK TO THIS UNIQUE MOUNTAIN WORLD.

IT'S NO COINCIDENCE THAT "SIERRA MADRE" MEANS "MOTHER MOUNTAINS."

HER GREAT RANGES NURTURE AN INCREDIBLE DIVERSITY OF LIFE AND GAVE BIRTH TO SOME OF THE GREATEST CIVILIZATIONS ON EARTH.

THE AZTECS BELIEVED MOUNTAINS BROUGHT YOU CLOSER TO THE HEAVENS.

IN MEXICO'S MOTHER MOUNTAINS, IT'S EASY TO SEE WHY.

THIS PROGRAM IS AVAILABLE ON DVD TO ORDER VISIT SHOP.PBS.ORG OR CALL 1-800-PLAY-PBS ALSO AVAILABLE ON ITUNES.