HE SAT DOWN WITH LINSEY DAVIS. >> I WILL NOT FIGHT YOU, BROTHER! >> I'M NOT YOUR BROTHER. >> Reporter: AUDIENCES ARE NO STRANGER TO HEART-POUNDING ACTION AND ADVENTURE WHEN CHRIS HEMSWORTH IS ON THE SCREEN. >> FORGIVE ME, JANE. >> Reporter: NOW THE STAR OF MARVEL'S "THOR" IS TAKING ON HIS TOUGHEST CHALLENGE YET. >> YOU'RE PROBABLY ASKING YOURSELF WHY I'M DANGING OFF A ROPE 1,000 FEET OFF THE GROUND. I'M ASKING THE SAME QUESTION. >> Reporter: ON THE NEW NAT GEO SERIES "LIMITLESS," HEMSWORTH PUSHES HIMSELF TO HIS MENTAL AND PHYSICAL LIMIT, ALL IN THE NAME OF LONGEVITY. NATIONAL GEOGRAPHIC IS A DIVISION OF DISNEY, ABC'S PARENT COMPANY. >> WHAT ARE YOU THINKING? >> OH, NOTHING, JUST TRYING TO ENJOY THE VIEW. BE COMFORTABLE IN A VERY UNCOMFORTABLE SITUATION. >> HOW ARE YOU APPROACHED ABOUT DOING "LIMITLESS"? >> DARREN ARONOFSKY CALLED ME UP AND SAID, I WANT TO DO THIS DOC SERIES ON LONGEVITY. I'M PRETTY HEALTH CONSCIOUS THROUGH MY LIFE. A PRETTY GOOD SENSE OF NUTRITION. IN THE SPACE OF LONGEVITY, IT WAS ALL INCREDIBLY NEW. AND UNKNOWN TERRITORY FOR ME. >> WAS THERE ANY HESITATION? >> I DON'T THINK I KNEW WHAT THE SHOW WAS WHEN I WAS FIRST ASKED TO DO IT. INITIALLY IT WAS ABOUT COLD WATER IMMERSION, DOING ICE BATHS. BEFORE I KNOW IT, WE'RE FLYING INTO NORWAY AND I'M SWIMMING IN THE ARCTIC OCEAN. >> WHAT WAS THE MOST CHALLENGING? >> EACH EPISODE WAS CHALLENGING AND TOUGH IN DIFFERENT WAYS. THE FOUR-DAY FAST, WATER-ONLY FAST, WAS VERY INTENSE. ESPECIALLY I COME OFF SHOOTING "THOR" WHERE I WAS CONTINUOUSLY EATING, HAVING AS MANY CALORIES AS I COULD. CLIMBING THE ROPE. >> WHAT I'M PROPOSING IS DANGLE AT THE BOTTOM, CLIMB ALL THE WAY UP. >> INTIMIDATED. >> THE SHOCK EPISODES, SWIMMING IN NORWAY IN FREEZING CONDITIONS, THAT FELT LIKE, YOU KNOW, A THOUSAND ICE CREAM HEADACHES. A LOT OF IT BECAME MENTAL FORTITUDE AND TRUSTING THE SCIENCE AND THE PEOPLE THAT WERE GUIDING YOU THROUGH THESE EXPERIENCES. KIND OF JUST, HEAD DOWN AND GO FOR IT. >> IF YOU PUSH YOURSELF BEYOND THE POINT YOU SHOULD BE COMING OUT, THERE'S REAL RISK YOU COULD DIE. >> THE BASIC IDEA HERE IS THAT THERE'S SO MUCH NEW SCIENTIFIC RESEARCH ABOUT HOW YOU CAN BEST UNLOCK YOUR OWN SUPERPOWERS, RIGHT? WHAT DID YOU LEARN ABOUT THAT UNLAWFUL LIKE, HM, THERE MIGHT ACTUALLY BE SOMETHING HERE? >> EVERY DAY WAS AN EDUCATIONAL EXPERIENCE FOR ME. AND I CONTINUE TO DO A LOT OF THINGS I LEARNT THROUGHOUT THE SHOW. BEING ABLE TO SHARE THAT ON A LARGER PLATFORM WITH MANY PEOPLE AND HAVE IT BE SOMETHING THAT IS EASILY ACCESSIBLE, TO HAVE SORT OF ME BE THE GUINEA PIG THROUGHOUT AND HAVE PEOPLE GO, OKAY, COOL. WE ALL HAVE THE SAME CHALLENGES AND STRESSES. >> WAS THERE ANY ASPECT THAT YOU'RE LIKE, I'M A REAL-LIFE SUPERHERO? >> NO, I DON'T HAVE THE TIME I WAS GETTING TO THE END, "DID WE GET THE SHOT? I'M NOT DOING IT AGAIN." >> Reporter: DESPITE HIS SEEMINGLY SUPERHUMAN ABILITIES, THE STAR'S BIGGEST DISCOVERY WAS A SHOCKING HEALTH SECRET HIDDEN IN HIS DNA. >> WHEN I FIRST STARTED EXPLORING HOW TO LIVE BETTER FOR LONGER, I WAS SENT FOR A WHOLE LIST OF TESTS. BY LONGEVITY EXPERT DR. PETER ATIA. >> SO WE DID SORT OF THE MILLION-DOLLAR WORKUP ON YOU. AND OVERALL, YOUR LABS LOOKED FANTASTIC. BUT WE DID FIND A COUPLE OF THINGS THAT ARE A CAUSE FOR CONCERN. YOU HAVE AN INCREASED RISK OF ALZHEIMER'S DISEASE. >> HM. HOW MUCH HIGHER? >> PROBABLY SOMEWHERE BETWEEN 8 AND 10 TIMES HIGHER THAN THAT OF SOMEONE IN THE GENERAL POPULATION. >> YEAH, RIGHT, OKAY. I DON'T KNOW, YOU'RE CONSTANTLY THINKING YOU'RE GOING TO LIVE FOREVER. ESPECIALLY AS A YOUNG INDIVIDUAL. AND THEN TO ALL OF A SUDDEN BE TOLD, OH, THIS MAY BE THE THING THAT MIGHT TAKE YOU OUT WAS LIKE, WHOA. >> BUT IT'S MY BELIEF THAT IF WE TAKE EVERY STEP POSSIBLE, WE CAN REDUCE YOUR RISK TO THAT OF ANYONE ELSE. THIS IS GOING TO MOTIVATE YOU TO TAKE STEPS TODAY THAT MOST PEOPLE YOUR AGE WOULD NEVER THINK ABOUT UNTIL THEY'RE IN THEIR 50s OR 60s. >> SURE. SURE. I'VE SPENT MOST OF MY LIFE PAYING VERY DETAILED ATTENTION TO MY HEALTH AND WELLNESS. BUT I'VE NEVER REALLY FOCUSED ON THE HEALTH OF MY BRAIN. THIS NEWS CHANGES THINGS. >> Reporter: AFTER LEARNING ABOUT HIS INCREASED LIKELIHOOD OF DEVELOPING ALZHEIMER'S, 39-YEAR-OLD HEMSWORTH SAYS HE'LL BE TAKING A BREAK FROM HOLLYWOOD FOR A WHILE, GOING HOME TO AUSTRALIA TO SPEND MORE TIME WITH HIS WIFE AND THREE CHILDREN, EMBRACING THE JOY OF FAMILY WITH NO LIMITS. >> LET'S GET ON WITH IT, SHALL WE? >> OUR THANKS TO LINSEY. YOU CAN B