How on earth did you get so interested in beavers?

I always loved wetlands when I was a kid.

And they were the coolest thing.

I was obsessed with them.

I didn't think there were any jobs in wetlands.

So I went to college and studied physics and chemistry.

O'BRIEN: She used her degree to land a job at the Los Alamos National Laboratory.

She worked on the team that maintains the nuclear weapons stockpile.

Wasn't really feeling that job.

Uh, it paid the student loans, but it wasn't necessarily my life's calling.

So I was feeling a little sad and sitting on my couch watching TV, and a documentary called "Leave It to Beavers" came on.

(theme playing) O'BRIEN: "Leave It to Beavers" was an episode of the PBS program "Nature" that aired in 2014.

(bellows) FAIRFAX: They are just showing all of these images of beaver dams in the desert, and everything is super-dry, except in the beaver pond.

And it's super-green and lush, and everything's alive.

And they were saying, "Especially as climate is changing, "there's so much we don't know about this animal, and we need more people to study this."

And I'm immediately hooked.

And, like, after a month of thinking nonstop about beavers, I decided to follow my passion and study beavers, because it felt hopeful, when everything else about climate change felt really, really grim.

That moment on the couch.

-Yeah.

How pivotal was that in your life?

-Looking back at it now, if I hadn't been feeling mopey and decided to watch TV on my couch, I may never have seen it.

I may never have thought about beavers.

♪ ♪ O'BRIEN: She took me to the place where she first started observing beavers ten years ago.

Well, what's the secret to having good balance in this situation?

-The secret to balance is... Field training is mostly doing yoga and one-leg poses so that you're good holding still when you're unsteady until you regain your composure and can take another step.

-Ah, so, downward dog means upward beaver.

-Exactly.

O'BRIEN: We were in Colorado, near Boulder, in a place called Left Hand Canyon.

It was time to get below the surface.

One by the lodge, one by the dam, one in the middle?

(Fairfax responding) O'BRIEN: Team member Nevé Baker suited up for some chilly snorkeling.

-(bleep) freezing!

-Is it... -Sorry.

Freezing, very cold.

(both laugh) O'BRIEN: Breathtaking in more ways than one.

♪ ♪ -It's magical down there.

♪ ♪ There are these, like, delicate canals that they've dug out where... They wouldn't exist if the beavers weren't actively maintaining them.

♪ ♪ O'BRIEN: The pond is filled with trout.

The fish that have evolved with beavers thrive in this environment and have no problem making their way through beaver dams.

Beavers create a ripple effect throughout the forest ecosystem.

The wetlands they build sustain a diverse array of wildlife, and their generosity doesn't stop there.

They often offer hospitality to other animals seeking shelter from the cold.

As Emily explained, there's a reason they are called "lodges."

They're more than just homes, they're havens.

The beavers want to have sort of a moat all the way around the beaver lodge.

The entrances are usually underwater.

And that first big chamber is usually where the beavers are going to eat their food, it's where they're going to groom each other, it's where they're going to hang out.

But sometimes they have a little tunnel and then another big chamber, and then that would be where they sleep.

The walls are usually actually one- to three-feet-thick on these.

So it's very, very insulated structures.

O'BRIEN: Come winter, you can see just how cozy it gets with a thermal camera.

FAIRFAX: And if it's full of all those beavers sleeping in their snuggle pile, the lodge might be sitting at 50, 60, even 70 degrees Fahrenheit when the rest of the landscape is, like, 30.

What you get is muskrats coming in, mice coming in, snakes coming in, frogs coming in-- everyone's coming in.

The beaver doesn't necessarily want to share its space.

So sometimes they'll make more rooms.

O'BRIEN: Let's say it got good reviews on Airbnb... -Mm-hmm.

-...and you had a lot of... -Yeah.

-...you had a lot of visitors.

How many rooms might there be?

If this was a top stop on Airbnbeave... -(laughs) -...then, inside, you could have, you know, four or five extra rooms with little visitors inside, but the beavers are going to maintain their space for themselves, and give everyone else their own little rooms.

O'BRIEN: Trekking through wetlands and building dollhouse-scale beaver dams are fun highlights, but the foundation of Emily's work happens here in her office in Minneapolis.

She and her team use satellite imagery and a computer model to determine where beavers are thriving and where they might be needed to improve the landscape.

FAIRFAX: What parts of this watershed, then, would be targets for vegetation restoration?

O'BRIEN: The overarching goal is to create a beaver census for North America to get a real handle on the size of the beaver nation and its potential impact as a climate change solution.

♪ ♪ Were you surprised at the number of beaver dams that are out there?

FAIRFAX: Oh, totally.

They are in abundance in some places.

A lot of people have this idea that beavers are gonna to take over and they're going to destroy a lot, but beavers are out there doing a huge amount of work and we don't even take notice.

You're making a nation of beaver believers, right?

Is that the goal?

(both laugh) I want a very beaver-literate nation... -Yeah.

-...that decides on their own that they believe.