NARRATOR: THE PACIFIC OCEAN IS FULL OF WONDER AND MYSTERY.

SYNCHRONIZED SWIMMERS THAT APPEAR OUT OF THE BLUE, AND A NAVIGATOR LIKE NO OTHER.

A RACE TO SAVE ANCIENT LIFE, AND LIFE THAT COMES BACK FROM THE DEAD.

MYSTIFYING MASTERPIECES... AND THEIR MAGNETIC APPEAL.

HERE WE FIND UNANSWERED QUESTIONS AND UNDISCOVERED TREASURES.

THIS IS A JOURNEY TO PLUMB THE OCEAN'S RIDDLES.

64 MILLION SQUARE MILES.

ONE-THIRD OF OUR PLANET.

THE BIGGEST OCEAN ON EARTH.

WITH SECRETS TO FATHOM.

[WATER LAPPING] [SONAR PINGING] NARRATOR: SEPTEMBER ON OSTIONAL BEACH IN COSTA RICA.

AND ONE OF THE GRANDEST MOMENTS IN NATURE'S CALENDAR BEGINS: THE ARRIBADA, A TURTLE NESTING TIDE.

OVER THE NEXT 10 DAYS, UP TO HALF A MILLION OLIVE RIDLEY SEA TURTLES WILL ARRIVE... ALL FEMALES.

AT JUST TWO FEET LONG, THEY'RE THE SMALLEST OF THE WORLD'S SEA TURTLES, BUT THE MOST ABUNDANT.

SOME MAY HAVE CROSSED HALF THE PACIFIC TO GET HERE.

IT'S A HOMECOMING.

LIKE ALL THE OTHERS, THIS YOUNG FEMALE HATCHED ON THIS BEACH.

BUT EVER SINCE, SHE'S BEEN AT SEA.

IN 12 YEARS, THIS IS THE FIRST TIME SHE'S LEFT THE WATER.

HOW SHE FOUND THIS BEACH AGAIN AND ARRIVED IN SYNCH WITH HER SISTERS IS NOT FULLY UNDERSTOOD.

THE JOURNEY OF THOUSANDS OF MILES ENDS A FEW YARDS AWAY, IN THE DRY SAND ABOVE THE HIGH TIDE.

SHE MUST DIG A HOLE TO PROTECT THE EGGS ONE TO TWO FEET DEEP.

IT CAN TAKE UP TO 3 HOURS FOR AN OLIVE RIDLEY TO TRAVEL UP THE BEACH TO LAY HER EGGS AND RETREAT.

SHE WILL HAVE MADE IT JUST OFFSHORE A SINGLE TIME AND STORED THE MALE SPERM WITHIN HER FOR USE THROUGHOUT THE NESTING SEASON, ENOUGH FOR HER TO JOIN 3 ARRIBADAS THIS YEAR AND LAY 3 CLUTCHES OF EGGS, AT LEAST 100 EGGS PER NEST.

BUT NOT ALL WILL SURVIVE.

AMERICAN BLACK VULTURES AND FERAL DOGS ALL FEAST ON FRESH OLIVE RIDLEY EGGS.

BUT NATURE FACTORS IN THE LOSSES.

ACROSS A BREEDING SEASON, THIS SINGLE BEACH WILL HARBOR UP TO A MILLION NESTS, UP TO 100 MILLION EGGS IN ALL, OVERWHELMING THE THREATS TO SURVIVAL WITH SHEER WEIGHT OF NUMBERS.

IT'S CALLED PREDATOR SWAMPING.

BETWEEN THE SIMULTANEOUS NESTING, THE ABUNDANCE OF NESTS, AND THE GLUT OF EGGS, ENOUGH YOUNG WILL SURVIVE TO CARRY ON THE SPECIES.

HOW THE SEA TURTLES NAVIGATE THE BIG PACIFIC IS A MYSTERY.

TO US, THE OCEAN SEEMS A MAPLESS BLUE.

BUT TO ITS INHABITANTS, IT'S FULL OF HIGHWAYS AND SIGN POSTS.

[SQUEAKING] [VOCALIZATIONS] THEY EVEN SEEM TO HAVE THEIR OWN GPS.

MALPELO ISLAND, COLOMBIA.

A FORMER VOLCANO ERUPTING ALMOST 12,000 FEET FROM THE DEEP.

LIKE OTHER PINNACLES IN THE OCEAN, IT ATTRACTS AN UNUSUALLY LARGE CROWD OF SHARKS-- HAMMERHEADS, SILKY SHARKS, WHALE SHARKS, SOME OF THEM DRAWN FROM THOUSANDS OF MILES AWAY.

THEY CONGREGATE FOR DAYS, WEEKS, EVEN MONTHS.

BUT WHY?

SOME SCIENTISTS SUSPECT THAT THE ISLAND SERVES AS A BASE CAMP.

EACH NIGHT, THEY LEAVE TO HUNT IN DEEPER WATER, THEN RETURN TO CIRCLE THE ISLAND, GLIDING ON THE CURRENTS THAT SWEEP PAST.

THE OTHER MYSTERY IS HOW DO THEY FIND THEIR WAY HERE?

THE ANSWER MAY LIE AT THE CROSSROADS OF GEOLOGY AND BIOLOGY.

SHARKS RIVAL ANY PHYSICS LABORATORY FOR SENSING TINY ELECTRICAL EFFECTS AS WEAK AS ONE BILLIONTH OF A VOLT.

ANIMALS FROM DOLPHINS TO BEES SHARE THE SAME TRAIT.

THESE HAMMERHEADS' BROAD NOSES HOLD SPECIAL ORGANS THAT CAN SENSE ELECTRICAL FIELDS LIKE THE FAINT ELECTRICAL SIGNATURES OF THEIR PREY-- STINGRAYS AND OTHER FISH HIDDEN BENEATH THE SAND.

WITH EVERY TWITCH OF A MUSCLE, WITH EVERY BEAT OF ITS HEART, THE PREY EMITS AN ELECTRICAL CHARGE THE SHARKS CAN DETECT.

LIKEWISE, THE EARTH HAS A POWERFUL MAGNETIC FIELD THAT TRAVELS ACROSS THE SURFACE FROM POLE TO POLE.

SCIENTISTS WHO STUDY SHARK MIGRATION SUSPECT THEY CAN DETECT THE EARTH'S MAGNETIC FIELDS JUST AS THEY DETECT ELECTRICAL FIELDS.

PINNACLES LIKE MALPELO MAY STAND OUT AS A MAGNETIC ANOMALY AND THUS A MAGNETIC LANDMARK, A BEACON FOR SHARKS.

SCIENCE IS STILL PLUMBING THE MYSTERY.

60 FEET BENEATH THE PACIFIC, ANOTHER ENIGMA HAS BEEN CRACKED.

[SONAR PINGING] IN 1995, DIVERS DISCOVERED DELICATE PATTERNS FURROWED INTO THE SEABED OFF JAPAN'S AMAMI ISLAND.

THEY WERE SHORT LIVED.

THE CURRENT WOULD SWEEP THEM AWAY, AND THEY'D REAPPEAR ELSEWHERE.

WHO, OR WHAT, DID THIS?

THE CRYPTIC DESIGNS BAFFLED AMATEUR EXPLORERS AND SEASONED SCIENTISTS ALIKE.

THE MYSTERY LINGERED FOR MORE THAN A DECADE.

THEN IN 2011, SCIENTISTS WATCHED THE SEABED FOR ALMOST A YEAR.

THEY DISCOVERED NOT JUST THE ARTIST, BUT A BRAND-NEW SPECIES.

THE WHITE-SPOTTED PUFFER FISH.

SO ELUSIVE, IT'S RARELY BEEN FILMED.

THE GOAL OF THIS 3-INCH ARTIST IS TO GATHER FINE SAND TO BUILD A BED FOR HIS MATE'S EGGS.

OTHER FISH BUILD NESTS, BUT NONE CREATE ART.

GATHERING FINE SAND IN A PERPETUAL CURRENT IS LIKE COLLECTING SMOKE IN THE WIND.

IN AN ENGINEERING TRIUMPH, THESE SAND CIRCLES REDUCE THE APPARENT CURRENT BY ALMOST 25%.

THE RADIAL SPOKES CHANNEL THE CURRENT TO SWEEP THE FINEST OF SAND INTO THE CENTER OF THE LOVE NEST.

THIS DILIGENT PLOWING LIFTS THE SAND TO BE CAPTURED IN HIS ARTFUL TRAP.

AS THE DESIGN EMERGES, IT'S MORE THAN 20 TIMES HIS SIZE.

TO REACH 6 FEET IN DIAMETER WILL TAKE MORE THAN A WEEK OF RELENTLESS WORK.

HE CAN'T REST BECAUSE THE SHIFTING TIDES THREATEN TO BLUR THE FINE DETAILS AND BLOW DEBRIS ONTO HIS CANVAS.

AND NOW THE FINAL TOUCHES.

THE PUFFER FISH VERSION OF FLOWERS AND CHOCOLATE: SAND DOLLARS AND SEASHELLS.

A FEMALE FINDS THE ARTIST'S GALLERY.

SHE SHOWS APPROVAL BY HOVERING IN THE CENTER.

MATING STARTS WITH A ROUGH PECK ON THE CHEEK AND ENDS IN THE BLINK OF AN EYE.

SECONDS IS ALL IT TAKES.

SHE DEPOSITS HER EGGS AND SHE'S GONE, NEVER TO RETURN.

THE ARTIST REMAINS WITH HIS CREATION AND HIS OFFSPRING.

THE EGGS MUST BE PROTECTED AT ALL COSTS.

HE FANS THE NEST TO REFRESH THE EGGS WITH OXYGENATED WATER AS THE CURRENT SLOWLY ERODES HIS MASTERPIECE.

WHEN THE EGGS HATCH, HE'LL BEGIN A NEW CYCLE OF ART AND LIFE.

IT TOOK 16 YEARS TO UNCOVER THE MYSTERY OF THE SAND CIRCLES, A DROP IN THE BUCKET OF TIME IN THE ANCIENT PACIFIC.

AT LEAST 5 MASS EXTINCTIONS HAVE EACH WIPED OUT 75% TO 95% OF LIFE ON EARTH.

EACH TIME, LIFE CLAWED ITS WAY BACK, INCLUDING A PREHISTORIC RELIC FOUND ONLY IN THE BIG PACIFIC.

[SONAR PINGING] THE CHAMBERED NAUTILUS.

UP TO 10,000 SPECIES OF NAUTILUS ONCE LIVED.

NOW THERE'S JUST 6...

THE LAST SURVIVORS OF A GROUP OF ANIMALS THAT THRIVED IN THIS OCEAN HALF A BILLION YEARS AGO.

NAUTILUSES ARE CEPHALOPODS, RELATIVES OF THE SQUID, THE CUTTLE FISH, AND THE OCTOPUS.

BUT THE NAUTILUS PREDATES THEM ALL.

ONE OF VERY FEW ANIMALS TO SURVIVE 5 MASS EXTINCTIONS ALMOST UNSCATHED AND LITTLE CHANGED.

HOW THIS 8-INCH WONDER SURVIVED IS THE FIRST MYSTERY.

HOW IT MOVES IS THE SECOND, BECAUSE THE CHAMBERED NAUTILUS IS ONE OF THE FEW SHELLED ANIMALS THAT CAN SWIM.

THE ANSWER LURKS IN ITS SHELL-- HOLLOW CHAMBERS, UP TO 30 IN AN ADULT.

IT CAN CONTROL ITS BUOYANCY THE WAY A SUBMARINE DOES, BY FILLING AND RIDDING THE CHAMBERS OF AIR-- AIR EXTRACTED FROM SEA WATER.

IT SCOOTS ALONG BY JETTING WATER FROM ITS HYPONOME FUNNEL, SCAVENGING OR HUNTING WHATEVER IT CAN FIND, TRACKING THE CURRENTS, FOLLOWING THE SCENT OF PREY AND CARRION WITH A NOSE-LIKE RHINOPHORE ORGAN.

BY NIGHT, IT HUNTS ON CORAL REEFS.

BY DAY, IT DIVES AS DEEP AS 2,000 FEET TO REST.

MAYBE THAT'S THE SECRET OF THE NAUTILUS'S SURVIVAL ACROSS EONS OF EXTINCTION: A REFUGE IN THE DEPTHS, FAR FROM THE DANGEROUS WORLD.

[SONAR PINGING] NARRATOR: SOME OF THE OCEAN'S OLDEST AND STRANGEST ANIMALS ARE STRUGGLING TO SURVIVE TODAY.

THE HORSESHOE CRAB DATES BACK 450 MILLION YEARS.

ALMOST TWICE AS OLD AS THE OLDEST DINOSAUR, IT HAS BARELY CHANGED.

WITH ITS HARD CARAPACE, IT COULD BE MISTAKEN FOR A CRUSTACEAN...

BUT IT'S ACTUALLY A CHELICERATE, THE FAMILY OF SPIDERS AND SCORPIONS.

HORSESHOE CRABS ARE THE BULLDOZERS OF THE SEA BOTTOM, PLOWING IN SEARCH OF MEALS THEY CAN SETTLE ON AND GRIND TO A PULP.

SMALL FISH, BIVALVES, ALGAE...

IT'S ALL GOOD.

BUT LIKE THE NAUTILUS, THEY LEAVE A FEW MYSTERIES IN THEIR WAKE.

THEIR BODIES ARE STUDDED WITH EYES.

FOR PRIMARY VISION, THEY USE A PAIR OF LARGE COMPOUND EYES CONTAINING SENSORY CELLS 100 TIMES LARGER THAN OURS.

UP TO 6 MORE EYES ON TOP OF THEIR SHELL CAN SENSE VISIBLE AND ULTRAVIOLET LIGHT.

TWO MORE EYES UNDER THEIR SHELL.

10 EYES IN ALL.

PLUS DOZENS OF LIGHT-SENSITIVE PHOTORECEPTORS ON THE TOP AND SIDES OF THEIR TAILS.

WHY DOES A BOTTOM FEEDER NEED SO MUCH VISUAL INFORMATION?

SCIENTISTS SUGGEST THEIR EXTREME SENSITIVITY TO LIGHT HELPS THEM TREK THEIR WAY TO THE SHORELINE, WHERE THEY GO TO LAY THEIR EGGS AFTER MATING.

HORDES OF HORSESHOE CRABS USED TO SWARM THIS CHINESE BEACH, BUT THOSE DAY ARE OVER.

SINCE 2002, SOME POPULATIONS HAVE DECLINED BY 90%.

A CREATURE THAT SURVIVED 5 WAVES OF EXTINCTION FINALLY MET A FORCE IT CAN'T WITHSTAND... INDUSTRIALIZATION.

[FREIGHTERS HONKING] POLLUTION, OVER-FISHING, AND DESTRUCTION OF MATING HABITATS.

THE MYSTERY NOW IS WHETHER A RADICAL EXPERIMENT CAN SAVE IT.

IT HINGES ON PROTECTING THEM WHEN THEY'RE MOST AT RISK.

HORSESHOE CRABS GROW SO SLOWLY, THEY TAKE ABOUT 12 YEARS TO MATURE.

AS THEY GROW, THEY REPEATEDLY SHED THEIR SHELLS.

EACH TIME, THEY'RE HELPLESS AGAINST PREDATORS.

FOR THE FIRST 5 YEARS, WHEN THEY ARE SMALL, THEY ARE PARTICULARLY VULNERABLE.

IN THE LAB, CRABS DON'T BURN ENERGY LOOKING FOR FOOD.

THE CONDITIONS ARE IDEAL, SO THE GROWTH RATE IS SPED UP.

IN ONE YEAR, THESE JUVENILES CAN ACHIEVE 5 YEARS OF GROWTH.

[WAVES CRASHING] A SHORTCUT TO SAFETY.

[INDISTINCT CHATTER] SCIENTISTS TEAM UP WITH SCHOOL KIDS TO RELEASE THE JUVENILES... FOSTERING THE NEXT GENERATION OF CRABS AND CONSERVATIONISTS.

[INDISTINCT CHATTER] JUVENILES REMAIN IN THE SHALLOWS FOR ALMOST A DECADE BEFORE HEADING TO DEEPER WATER AS ADULTS.

THE PROGRAM IS ONLY A FEW YEARS OLD, SO THE MYSTERY LINGERS-- CAN CHINA SAVE AN ANCIENT RELIC FROM MODERN TIMES?

[GULLS CALLING] NARRATOR: THESE SAME WATERS ARE HOME TO ANOTHER UNUSUAL YET TROUBLED CREATURE.

[DOLPHIN SQUEAKS] WHITE HUMPBACK DOLPHINS, FOUND IN THE PACIFIC AND INDIAN OCEANS FROM CHINA TO AUSTRALIA TO AFRICA.

THEY'RE NOT ALBINOS LIKE SOME WHITE WHALES.

THEY'RE BORN DARK GRAY... [DOLPHIN SQUEAKS] BUT SLOWLY LIGHTEN UNTIL THEY BECOME SNOWY WHITE.

AT TIMES, SOME EVEN APPEAR PINK, BUT SCIENTISTS BELIEVE THAT'S A HEAT RESPONSE THE WAY HUMANS FLUSH WHEN HOT.

WHILE THIS SPECIES CHANGES FROM DARK TO LIGHT, SOME OF ITS COUSINS CHANGE FROM LIGHT TO DARK.

EACH TRANSFORMATION AS MYSTERIOUS AS THE OTHER.

TO CHINESE FISHERMEN, THE WHITE DOLPHINS ONCE MEANT BAD LUCK.

TEEMING IN THE THOUSANDS, THEY VIED FOR THEIR CATCH.

NOW THE POPULATION HAS PLUNGED TO 2,500 AND FALLING.

CHINA HAS GIVEN THE DOLPHINS THEIR HIGHEST LEVEL OF PROTECTION, BUT IT MAY NOT BE ENOUGH.

TODAY, TOURISTS TAKE CRUISES HOPING TO SPY A WHITE DOLPHIN, BUT THEIR CHANCES OF SUCCESS ARE DWINDLING.

THE DOLPHINS ARE NOW SO RARE THAT GLIMPSING A WHITE HEAD ABOVE THE WAVES HAS BECOME A GOOD OMEN EVEN FOR FISHERMEN.

WHERE THERE'S DOLPHIN, THERE'S FISH.

THE WESTERN PACIFIC HARBORS ANOTHER CREATURE WITH ITS OWN MYSTERIOUS COLOR.

[GULLS CALLING] [SONAR PINGING] NARRATOR: BENEATH THE SPRING FULL MOON, FISHERMEN AWAIT A VAST MIGRATION IN THE CHANNELS BEYOND TOYAMA BAY-- A SPECIAL EVENT THEY HAVE RELIED UPON FOR GENERATIONS, MAKING IT AN HONORED PART OF THEIR CULTURAL TRADITIONS.

[GULLS CALLING] PULSING FROM THE DEPTHS, UP TO A BILLION FIREFLY SQUID, EACH NO LONGER THAN YOUR FINGER.

THEY GLOW LIKE THE STARS.

THE FIREFLY SQUID BOASTS A TYPE OF LUMINESCENCE LIKE NO OTHER ON THE PLANET.

AT THE HEART OF EACH BRIGHT BLUE DOT IS A CRYSTAL OF PROTEIN... RADIATING LIKE AN OCEAN SAPPHIRE.

PERHAPS THE SQUID HITS THE HIGH BEAMS TO INTIMIDATE A PREDATOR OR COMMUNICATE WITH OTHER SQUID.

EITHER WAY, SCIENTISTS ARE STUMPED.

LIKE A FAIRY TALE, THE STARS AND MOON SHINE DOWN, AND THE FIREFLIES SHINE BACK.

AND THE FISHERMEN GATHER.

FOR CENTURIES, FIREFLY SQUID HAVE BEEN A JAPANESE DELICACY-- FRIED, BOILED, OR RAW.

ALL THE BETTER IF THEY STILL GLOW AS THEY HIT THE PLATE.

THE FISHERMEN ARE RUNNING AGAINST THE CLOCK TO ENSURE THEIR CATCH IS FRESH BECAUSE ALL THESE SQUID ARE PROGRAMMED TO DIE AFTER SPAWNING.

AND IT'S THAT TIME.

EVERY ONE IS A FEMALE.

AS THEY RISE, THEY RELEASE THEIR FERTILIZED EGGS... BEADED NECKLACES A YARD LONG.

WITH UP TO 20,000 EGGS PER CHAIN, EACH EGG IS SO TINY, IT SLIPS THROUGH THE FISHERMEN'S NETS.

IN 8 OR 9 DAYS, THEY'LL HATCH IN THE BIG PACIFIC.

AND A YEAR FROM NOW, THEY WILL RETURN TO SWARM THIS BAY, MULTIPLY, AND DIE.

COMPLETING A LIFE CYCLE AS BRIEF AS IT IS BRILLIANT.

FAR TO THE SOUTH LIES ANOTHER DAZZLING MYSTERY...

BUT THIS ONE IS MAN-MADE.

[SONAR PINGING] NARRATOR: THE FEDERATED STATES OF MICRONESIA-- AN ARCHIPELAGO OF MORE THAN 600 ISLANDS ALONG THE EQUATOR.

ONE REMOTE ISLAND HOLDS A SITE UNRIVALED IN THE ENTIRE PACIFIC OCEAN-- AN ANCIENT CITADEL CALLED NAN MADOL.

AN ALMOST MYTHICAL PACIFIC DYNASTY KNOWN AS THE SAUDELEURS RULED THE ISLAND OF POHNPEI FOR ABOUT 500 YEARS... THEN VANISHED.

THEY LEFT BEHIND A TROPICAL VENICE-- A CITY OF 92 STONE BUILDINGS WITH WALLS UP TO 50 FEET HIGH, ERECTED ON CORAL ISLETS LINKED BY BRIDGES SPANNING 200 ACRES.

DESIGNED PERHAPS AS A BASTION AGAINST MONSOONS, OR MORTALS, HOW THEY BUILT IT BOGGLES THE MIND.

A STONE-AGE CULTURE TRANSPORTED THREE-QUARTERS OF A MILLION TONS OF BASALT FROM ACROSS THE ISLAND, WEIGHING AS MUCH AS THE GOLDEN GATE BRIDGE.

NO MACHINES, YET THEY MOVED COLUMNS WEIGHING AN AVERAGE OF 5 TONS, SOME UP TO 50 TONS.

TODAY'S ISLANDERS SAY THE ROCK PILLARS WERE FLOWN HERE BY SORCERERS-- THE TWIN BROTHERS WHO FOUNDED THE DYNASTY.

THE TRUTH IS LOST IN TIME, BUT LOST DOESN'T ALWAYS MEAN GONE.

IN THE BIG PACIFIC, THE PAST SOMETIMES MYSTERIOUSLY REEMERGES JUST WHEN WE THOUGHT IT WAS DEAD.

[SONAR PINGING] NARRATOR: 350 MILES OFF THE COAST OF AUSTRALIA, A LONE ISLAND STANDS VIGIL.

TALLER THAN THE EMPIRE STATE BUILDING, BALL'S PYRAMID IS A FORBIDDING SPIRE.

IT RESISTS HABITATION AND DEFIES EXPLORATION.

IT'S HOME TO A SPECIES AS LONELY AND MYSTERIOUS AS THE PYRAMID ITSELF.

BUT THIS STRANGE TALE BEGINS 13 MILES AWAY ON ANOTHER ISLAND.

LORD HOWE ISLAND IS SO REMOTE, IT SAT UNDISCOVERED UNTIL 1788.

IT'S A GARDEN PARADISE THAT ERUPTED FROM THE OCEAN 7 MILLION YEARS AGO... [ANIMALS CHITTERING] NEVER PART OF ANY LANDMASS.

INHABITED BY A UNIQUE POPULATION OF BIRDS, LIZARDS, AND INSECTS... EACH TRANSPORTED TO THE ISLAND BY WIND, WAVES, OR WING.

[ANIMALS CHITTERING] ONE OF THE STRANGEST WAS THE LORD HOWE STICK INSECT.

THE TREE LOBSTER, AS EARLY SAILORS CALLED IT.

6 INCHES LONG, HEAVY, SOLID, AND SLOW...

DINING UNFETTERED ON THE LUSH VEGETATION UNTIL A SHIPWRECK IN 1918 RELEASED RATS.

THEY GORGED THEIR WAY THROUGH THE LORD HOWE FORESTS, EATING EVERY HELPLESS TREE LOBSTER THEY FOUND.

A CREATURE MILLIONS OF YEARS OLD GONE IN TWO YEARS.

[FLY BUZZES] NARRATOR: 1920 WAS THE LAST RECORDED SIGHTING OF A LIVING LORD HOWE STICK INSECT.

BUT THEY SURVIVED...

IN RUMOR, HINTS THAT THEIR STORY WASN'T OVER.

THROUGHOUT THE YEARS, EXPLORERS WHO TRIED TO CONQUER BALL'S PYRAMID CLAIM THEY SAW THE CORPSES OF STICK INSECTS RECENTLY DEAD.

COULD THE PYRAMID HARBOR A POPULATION OF CASTAWAYS?

ALMOST 80 YEARS AFTER IT WAS LOST, THE LIVING INSECT WAS REDISCOVERED BY ROCK CLIMBERS IN 2001... CLINGING TO A CLIFF OVER 200 FEET ABOVE THE PACIFIC.

THE SPECIES' TERRITORY?

A SINGLE BUSH.

IT'S POPULATION?

JUST 24 ANIMALS.

ODDS OF SURVIVAL HERE?

1 MILLION TO 1.

A SINGLE STORM COULD HAVE WIPED OUT THEIR HOME.

IN A BEHAVIOR RARE AMONG INSECTS, TREE LOBSTERS PAIR OFF IN BREEDING COUPLES THAT SEEM MONOGAMOUS, AND THEY PRODUCE EGGS PROLIFICALLY.

THANKS TO INTENSIVE BREEDING AT ZOOS AROUND THE WORLD, EGGS FROM THOSE 24 SURVIVORS MULTIPLIED... AND IN 15 YEARS BECAME MORE THAN 13,000 TREE LOBSTERS.

THE SPECIES IS NOW RESCUED... A LAZARUS SPECIES, BACK FROM THE DEAD.

BUT HOW DID IT GET TO THESE ISLANDS AT ALL?

IT TRAVELED 13 WATERY MILES TO BALL'S PYRAMID FROM LORD HOWE.

BUT THE LARGER MYSTERY IS HOW DID THE STICK INSECTS GET TO LORD HOWE ISLAND IN THE FIRST PLACE?

ITS DNA SUGGESTS IT TRAVELED ALMOST 400 MILES FROM AUSTRALIA, MAYBE ON DRIFTWOOD... A JOURNEY AS EXTRAORDINARY AS IT IS INEXPLICABLE.

THE TREE LOBSTER HAS BEEN DISCOVERED TWICE.

IN THE 21st CENTURY, THE PACE OF DISCOVERY UNVEILS MORE MYSTERIES.

[SONAR PINGING] A WORLD LEADER IN DEEP OCEAN EXPLORATION PREPARES FOR A JOURNEY TO FIND THE OLDEST LIVING CREATURES ON EARTH.

PHIL NUYTTEN IS THE MARINE VERSION OF A WRIGHT BROTHER.

THEY TOOK US TO THE HEAVENS.

HE TAKES US INTO THE ABYSS.

FOR 40 YEARS, PHIL HAS ENGINEERED SUBMERSIBLES AND HARD SUITS USED BY EVERYONE FROM NASA TO THE U.S. NAVY TO TREASURE HUNTERS.

HE AND HIS MACHINES HAVE MADE SOME OF THE WORLD'S DEEPEST DIVES...

REVEALING THINGS WE'VE NEVER SEEN BEFORE.

[MAN SPEAKING INDISTINCTLY OVER RADIO] ROGER THAT.

ROGER THAT.

MAN: FLOATING TANKS PROCEED TO BOTTOM.

NARRATOR: THE "AQUARIUS" SUBMERSIBLE CAN CARRY 3 PEOPLE TO A DEPTH OF 1,000 FEET.

NUYTTEN: MY URGE HAS ALWAYS BEEN TO GO DEEPER AND STAY LONGER AND SEE THINGS THAT ARE NOT NORMALLY SEEN OR SELDOM SEEN.

NARRATOR: A PLACE AS ALIEN TO US AS ANOTHER PLANET.

MAN: ROGER.

CABIN PRESSURE IS 0.

02--21 DECIMAL 0.

OVER.

NUYTTEN: ROGER THAT.

NARRATOR: THE PRESSURE ON THE CRAFT QUICKLY DOUBLES.

THE RISK TO THOSE INSIDE MULTIPLIES WITH IT.

BEYOND 400 FEET, A PLACE THAT SUNLIGHT BARELY TOUCHES.

IT WAS ONCE THOUGHT THAT ALMOST NOTHING COULD LIVE HERE, BUT WE WERE WRONG.

THIS ALIEN AND SUNLESS WORLD IS HABITABLE.

OUR DESTINATION IS ANOTHER 100 FEET DEEPER.

AT THIS DEPTH, EVERY SQUARE INCH OF THE "AQUARIUS" BEARS THE WEIGHT OF MORE THAN 200 POUNDS OF SEA WATER.

PHIL'S GOAL?

A REEF MORE THAN 20 MILES LONG... HOME TO A RARE SPECTACLE, ONE OF THE FIRST CREATURES TO EVER INHABIT THE EARTH.

GLASS SPONGES.

THEY'VE LIVED ON EARTH MORE THAN 500 MILLION YEARS.

100 TIMES OLDER THAN MANKIND'S EARLIEST ANCESTOR, PERHAPS THE ANCESTOR OF ALL COMPLEX ANIMALS.

THIS IS WHAT EARTH'S FIRST MULTICELLULAR LIFE FORMS LOOKED LIKE.

LIKE THE TREE LOBSTER, THEY WERE THOUGHT EXTINCT.

WE KNEW THEM ONLY FROM THE FOSSIL RECORD, UNTIL A DEEP DIVE IN 1987 TURNED UP LIVING FOSSILS.

NUYTTEN: WE DIDN'T KNOW A WHOLE LOT ABOUT THEM WHEN I FIRST STARTED SCUBA DIVING 'CAUSE, OF COURSE, THEY ARE FAIRLY DEEP.

YOU SEE THEM IN CLUMPS IN VARIOUS PLACES AROUND HERE, BUT TO GET DOWN TO A SPONGE REEF, ABSOLUTELY INCREDIBLE.

NARRATOR: EXPLORERS LIKE PHIL HAVE FOUND RARE REMNANT POPULATIONS OF GLASS SPONGES IN ALL OUR OCEANS.

THEY'RE FILTER FEEDERS, SIPHONING SILICA FROM SEA WATERS TO PRODUCE THEIR NAMESAKE GLASS SKELETONS.

THIS REEF IS 4 STORIES HIGH.

LIVING SPONGES GROWING ON TOP OF DEAD ONES.

AND THE LIVING SPONGES HERE IN BRITISH COLUMBIA CAN BE UP TO 6,000 YEARS OLD.

NUYTTEN: SO THE SPONGE REEFS THAT WE HAVE HERE ARE QUITE UNIQUE IN THEIR BREADTH AND DEPTH AND THE LENGTH OF TIME THAT THEY'VE BEEN AROUND.

NARRATOR: SCIENTISTS HAVE FOUND GLASS SPONGES IN THE SOUTH CHINA SEA THAT ARE 11,000 YEARS OLD...

THE LONGEST LIFESPAN OF ANYTHING ON EARTH.

THE PACIFIC IS SO VAST, UNFATHOMABLE MYSTERIES TURN UP EVEN IN ITS SHALLOWS.

[SONAR PINGING] SABAH, IN MALAYSIA.

SOME OF THE WORLD'S RICHEST RAIN FORESTS RUN TO THE WATERLINE... WHERE A NEW NATURAL WONDER BEGINS.

A CORAL REEF SYSTEM OF IMMEASURABLE RICHNESS... THAT HIDES A MYSTERIOUS GRAVEYARD.

THROUGH A DARK CLEFT INTO A MAZE OF DROWNED CAVES, A PERILOUS JOURNEY FOR EVEN EXPERIENCED DIVERS.

60 FEET DOWN AND THEN MORE THAN 200 FEET INTO THE HEART OF THE ISLAND.

THE TOMB OF THE LOST TURTLES.

ONE TURTLE MIGHT WANDER IN AND NEVER WANDER OUT...

BUT DOZENS?

AFTER VISITING IN 1998, UNDERSEA EXPLORER JACQUES COUSTEAU LIKENED IT TO A GRAVEYARD FOR AFRICAN ELEPHANTS... A PLACE WHERE THEY GO TO DIE.

BUT AFRICA'S ELEPHANT GRAVEYARDS ARE A MYTH, AND SEA TURTLES DON'T GATHER TO DIE.

THIS IS THE ONLY ONE WE KNOW OF.

OTHER CLUES ONLY CLOUD THE MYSTERY.

DOZENS OF SHELLS LAY IN THE SAND.

THEY'RE ALL THAT'S LEFT OF STILLBORN OFFSPRING.

BUT FEMALES LAY THEIR EGGS ON BEACHES, NOT IN CAVES.

THE WATER IS PITCH BLACK, SO WE MIGHT GET LOST IN HERE.

BUT TURTLES?

THEY CAN HOLD THEIR BREATH UP TO 7 HOURS, AND THEY'RE ONE OF THE BEST NAVIGATORS IN THE SEA.

HOW DO THEY MANAGE TO CROSS THOUSANDS OF MILES OF THE BIG PACIFIC, YET NOT FIND THEIR WAY OUT OF HERE?

THE CRYPT REMAINS EVER CRYPTIC.

AFTER 50 DAYS UNDER THE HOT COSTA RICAN SUN, LIFE EMERGES.

SEA TURTLES TAKE THEIR FIRST BREATHS AND EXERCISE THEIR FLIPPERS FOR THE FIRST TIME.

TO SURVIVE, THEY MUST GET TO THE SEA...

BUT THEIR DESTINY WAITS FOR THEM ON THE SAND.

BIRDS HAVE GATHERED.

IT'S A RELIABLE FEAST.

THE HATCHLINGS CRAWL FROM THE SAND AT THE MOST DANGEROUS MOMENT IN THEIR ENTIRE LIVES.

[BIRDS SQUAWKING] LESS THAN 8% MIGHT MAKE IT TO THE OPEN OCEAN, BUT THAT'S STILL A MULTITUDE.

NOW THEIR MOTHER'S DRIVE TO NEST IN THE TENS OF THOUSANDS PAYS OFF.

BUT IT'S A MAD SCRAMBLE TO OBLIVION.

BECAUSE ONCE THESE LITTLE TURTLES HIT THE SEA, THEY VANISH FROM US.

SCIENCE IS ONLY BEGINNING TO UNDERSTAND THEIR LIFE'S JOURNEY.

SOMEWHERE, WE KNOW THEY LIVE AND THRIVE, BUT WHERE HAS NEVER BEEN DISCOVERED.

IF THIS VOYAGER SURVIVES, SHE'LL RETURN ONE DAY TO THIS SAME BEACH.

BETWEEN NOW AND THEN, HER LIFE, LIKE SO MUCH OF THE BIG PACIFIC, REMAINS A MYSTERY.

- [Announcer] Next time on Big Pacific.

A fight for dominance leads to unknown territory.

A secluded island with 20,000 traps.

And the Earth erupts with destruction.

The stakes are high in the violent Pacific.

(dramatic music) "BIG PACIFIC" IS AVAILABLE ON BLU-RAY AND DVD.

THE COMPANION BOOK IS ALSO AVAILABLE.

TO ORDER, VISIT SHOPPBS.ORG OR CALL 1-800-PLAY-PBS.

THIS PROGRAM IS ALSO AVAILABLE FOR DOWNLOAD ON ITUNES.