Millions of monarch butterflies flutter to the mountains in Mexico every October

In 1996, around one billion monarchs wintered there, but in 2013, a little less than 50 million monarchs traveled there.
6:38 | 02/09/18

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Transcript for Millions of monarch butterflies flutter to the mountains in Mexico every October
ancestors making history. The monarch butterfly, one, two, a bunch. Now listen. Now listen. That's the sound of butterflies, millions of butterflies fluttering through sunlight they come here in Mexico every year in one of the most in cred migrations on Earth. Flying thousands of miles to get here. It's a hazy morning. We're hiking up elrosario reserve to witness this mysterious miracle. It's getting thin zbler now under threat. Just need to catch my breath. We're at 9,000 feet above sea level. After hard tracking we finally spot them those dark patches. Those big black clumps are butterflies and they're still sleeping. Millions of sleeping butterflies huddled together for warmth. Trademark color owning. Orange. When it is 60 degrees they wake up, stretch their wings, shivering in cold not fear. She's shivering. Because it's still little chilly. Yeah. You're witnessing extraordinary access. Tourists are not allowed in this far. We're only here because we're with the monarch big wig for the world wild life fund. They come from as far away as Winnipeg, Canada. They weigh less than an ounce. That's over 2,000 miles clear across the U.S. For passenger plane eight hour flight for monarchs up to two months. To within few hundred feets of where their ancestors went the year before and year before that. Is he still on my head. Yeah. Why does this happen. The weather is the key. They escape from winter in Canada and states. Escaping the winter. How's it happen? No one really knows. How the monarchs come to fly a great distance is a mystery. Monarchs in your yard are short lived but in the northern U.S. And Canada a super hero generation, bigger, stronger, capable of living eight months and traveling across the continent. That's my hand. Sense of scale. Once of a year this freak generation is born that makes this migration. Exactly. They arrive just before the day of the day if dead in October, legend has it they are the spirits of the departed. Talking 500 years but not until 1975 when scientists proved they came from so, so far away, how is still the mystery they're trying to solve. Monarch watch tags the insects as they head south. They estimate about 80,000 were tagged in 2017 of which only 1% will be recovered. Look. We found a tag. Against the odds we found one, called it in to hq. Number is xut 121. The very next morning we skyped with Pam Byrd a tagger in laky, Texas. 700 miles away. Hello I'm Nick this is Eduardo. Nice to meet you Pam. Nice to meet you both. We found one of your butterflies. I know I can't believe this I feel like I won the lottery. Thank you for yr job it's very, very important this data. Thank you for your participation on your end as well. It's just magic that we can work together like this. Another slice of magic monarchs drinking from a stream where tourists mingle with them. I mean this is amazing. It's beautiful out here. I feel like I'm in heaven. Just being out here seeing these is cool, you're one with them. 1996 is the year with the largest number of monarchs here covering 44 acres of forest. And this is the biggest colony of monarchs in the world. In the world, yes. This is the most important spot for monarchs right now. In 2013 monarchs covered less than two acres, the lowest area in recorded history. Experts say it's partially due to climate change more hornet canes on the migration route and freak snow storm in 2016 that killed millions. Illegal logging is also a problem exer if. Enter Eduardo, he rallied the locals teaching them preserving the monarchs could be more profitable and rewarding. ??? ING trees to reforest and mushrooms to sell. Problem is now in the U.S. Herb sides killing milk weed plants along the route, monarchs will only lay their eggs on milk weed if you live on those routes you could make a difference, not only the government, the people. Are you going to go plant milk weed in your yard. We have lot S in our yard. Back in the forest Eduardo is blowing on a monarch to warm her up. Yeah, well done. You saved one. One. I'm trying to save million of them. For "Nightline" if in michoacan, Mexico. Up next, a lesson in courage

This transcript has been automatically generated and may not be 100% accurate.

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