>> YES, PLEASE HEED THOSE WARNINGS. ROB MARCIANO AND THE TEAM IN PLACE TO COVER THIS. ROB, THANK YOU. >>> IN THE MEANTIME, WE TURN TO THE STUNNING IMAGES JUST COMING IN TONIGHT, THE WORLD'S LARGEST ACTIVE VOLCANO IN HAWAII ROARING BACK TO LIFE. AND THE URGENT WARNINGS AT THIS HOUR FOR RESIDENTS THERE. MAUNA LOA ON HAWAII'S BIG ISLAND ERUPTING FOR THE FIRST TIME IN 38 YEARS. A THERMAL CAMERA, LOOK AT THIS, CAPTURING THE MOMENT IT ERUPTED. LAVA POOLING IN THE CRATER RIGHT AT THE TOP. OUR CHIEF NATIONAL CORRESPONDENT MATT GUTMAN WITH THE PICTURES COMING IN NOW. >> Reporter: TONIGHT, NEW HELICOPTER VIDEO CAPTURING MAUNA LOA'S FIERY AWAKENING, ERUPTING FOR THE FIRST TIME 38 YEARS. THE WORLD'S LARGEST ACTIVE VOLCANO ROARING BACK TO LIFE, AFTER IT'S LONGEST DORMANT PERIOD ON RECORD. >> YOU CAN CLEARLY SEE THAT THAT'S LAVA COMING DOWN MAUNA LOA. IT IS NUTS. >> Reporter: THERMAL CAMERAS SHOWING THE MOMENT IT ERUPTED, SPEWING VOLCANIC GAS INTO THE SKY AND SENDING LAVA GUSHING OUT FROM THE SUMMIT CRATER. TIME-LAPSE VIDEO ALSO CAPTURING THE BLAST OF ASH, DEBRIS, AND MOLTEN ROCK. LAVA STREAMS ARE FLOWING DOWN PART OF THE VOLCANO, BUT AUTHORITIES SAY THEY ARE NOT THREATENING COMMUNITIES DIRECTLY. BUT THERE IS CONCERN OVER FALLING ASH. PEOPLE WITH RESPIRATORY ILLNESSES WARNED TO STAY INDOORS, AND ANYONE GOING OUTSIDE TOLD TO WEAR A MASK. SHELTERS BEING OPENED AS A PRECAUTION. >> YOU KNOW, WE'RE WAY OVERDUE. WE WERE WAY OVERDUE FOR A MAUNA LOA ERUPTION, AND SO WE HAD PREPARED. >> Reporter: SOUTHWEST AIRLINES NOW CANCELING SEVERAL FLIGHTS TO AND FROM THE BIG ISLAND. MAUNA LOA MAKES UP MOST OF THE BIG ISLAND OF HAWAII, AND 50 MILES DOWN FROM ITS SUMMIT IT CONNECTS WITH HAWAII'S OTHER MAIN VOLCANO, KILAUEA. KILAUEA'S ERUPTION IN 2018 DESTROYED ABOUT 700 HOMES AS WE SAW FIRST-HAND BACK THEN. YOU CAN SEE ALL THAT LAVA FOUNTAINING HERE. NOW THIS IS FISSURE 17. SOME OF THE FOUNTAINING HAS GONE HUNDREDS OF FEET IN THE AIR. WHAT YOU'RE SEEING OVER THERE AND THOSE ROARS ARE STEAM AND GAS FLYING OUT OF THESE VENTS. THAT STUFF IS 2,000 DEGREES. AUTHORITIES SAY LAVA FROM TODAY'S ERUPTION IS CONTAINED TO THE SUMMIT AREA BUT ALERTED COMMUNITIES AT POTENTIAL RISK TO REVIEW THEIR EVACUATION PROCEDURES. DAVID, VOLCANOLOGISTS TELL ME WE'VE LIKELY ALREADY SEEN THE WORST OF THE ERUPTIONS. THEY WILL LIKELY START TO TAPER OFF FROM HERE. THEY ALSO SAY ANOTHER BIT OF GOOD NEWS IS THAT ERUPTIONS HAPPENED IN THE NORTHEAST. THAT MEANS IT WOULD TAKES WEEKS OF CONTINUOUS GUSHING OF LAVA TO ACTUALLY REACH POPULATED AREAS OR INFRASTRUCTURE. >> SO HOPEFULLY A LOT OF TIME TO