This article is part of The State of Science, a series featuring science stories from public radio stations across the United States. This story, by Michelle Loxton, was originally published on KCLU.

There’s a spot on Highway 101 in Agoura Hills, it’s pretty inconspicuous. There’s brown and green rolling hills on either side of the highway. Homes are sprinkled here and there. And then a small metal gate that leads off on a hiking trail. You probably wouldn’t know it, but soon this spot will be the location of the world’s largest animal crossing.

This crossing will reconnect habitats that have been cut off from each other for three quarters of a century and it’ll do it over a highway that is constantly buzzing with cars — 300,000 pass by this spot every single day.

In this piece we’re going on a geography voyage — from the north side of the highway to the south, and up the hills, above the highway, to get the real view.

We’ll start here — there’s a big open space on the northern side of the highway. It’s at the entrance to Liberty Canyon and where I meet Beth Pratt.

“You have oak trees, a little creek area here. And we’re listening to, actually, an Anna’s hummingbird giving a little song for us that is actually resonating even over that, that noise of traffic,” Pratt said.

She is the California Regional Director for the National Wildlife Federation.

“For me what’s kind of remarkable, but also sad. It’s the last sixteen hundred feet of protected space on both sides of the freeway,” said Pratt.

Pratt describes this area like an hourglass with the highway being the middle point. But unlike an hourglass where sand, or wildlife in this case, would flow from one side to another, Highway 101 has been an almost impenetrable wall for the wildlife in this region for three quarters of a century.

“The wildlife were already sort of coming to this area. And what the National Park Service study has shown for at least the wildlife they have collared or tracking, like the mountain lions or or coyotes. They get here and they’re like, ‘Ah Ah, I’m not crossing this and they turn around’,” said Pratt.

It’s a cooler day when we meet up and Pratt is wearing a Christmas Sweater with Mountain Lion P-22 on the front.

“Who I call the Brad Pitt of the Cougar world,” Pratt said.

That’s right — a puma celebrity. “P” is for puma and “22” is because he was the twenty second puma to get a tracking collar. Pratt also has a tattoo of P-22 on her arm and carries around a cardboard cut-out of the big cat.

As you can tell he’s a big deal to her and really the inspiration for the world’s largest animal crossing.

“For me, what he did was get the public engaged, which is really important. The park service and others have been talking about the need for connectivity for a while. But it wasn’t something that resonated with people outside of the environmental or scientific world. But all of a sudden boom, you get this lonely, dateless, handsome bachelor show up in Griffith Park,” said Pratt.

So the story of P-22 is pretty remarkable. He was born somewhere near here, in the Santa Monica Mountains but ultimately made his way across two very busy highways, unhurt and undetected, and ended up in the center of L.A. in Griffith Park about 50 miles away. It was kind of a fluke that he made it there safely and so has been unable to leave since… living in the area for more than a decade now. The lonely, handsome mountain lion, as Pratt puts it, became a local celebrity and spurred the campaign for connecting wild spaces.

“I think anthropomorphizing… fine. We are animals. You know. P22 isn’t lonely like, we’re lonely, but you know, he’s discontented. And I think when we talk about him dating, pretty much everybody realizes I’m not talking about him, you know, getting in an Uber and using Tinder,” said Pratt.

Wrong or right, that image of the sad bachelor cougar was effective. Pratt and the National Wildlife Federation began publicly raising money for this project back in 2014. It was priced at $90 million and they raised the money through private philanthropy, a combination of big and small donations, and from funds from the state Wildlife Conservation Board.

Now P-22 has been a huge inspiration for this project but he won’t benefit from this particular wildlife crossing – he’s just too cut off at the moment. But so many other mountain lions will.

Pratt thinks these types of wildlife crossings are the future of animal conservation.

“You don’t need a Yosemite on every block, but you need connectivity to open space habitats,” said Pratt. “Even in our best protected places on the planet, like a Yosemite or Yellowstone. Both places, you know, I had the good fortune to work in, the wildlife aren’t doing great, you know, in some respects, not all of them, but I mean, look at Yellowstone with the wolves. They walk an inch outside the boundary, which, you know, they don’t know park boundaries and they can be shot. Look at the bears in Yosemite. They don’t know the park boundaries.”

Pratt believes cordoning off wildlife into national parks hasn’t been really all that beneficial for them. Ironically, here in Southern California, we actually live in close proximity to and amongst a lot of wildlife. She believes that makes people care more.

“Separating us did not allow people then to build a day to day relationship with wildlife, and I think if anything, more than all the other stuff I mentioned, that’s key to ensuring wildlife have a future. Because if people don’t care about wildlife or don’t have relationships with it. How are you going to ask them to help save it?” said Pratt.

She is very excited for the day the crossing opens.

“I know I will just, you know, probably start crying once that first animal uses it and we’re taking bets like, what’s the first animal going to be to use it? Probably a lizard,” Pratt said.

Engineering never seen before

I make my way under the highway on a road that takes cars to the south side of the 101. There’s an echoey drum as cars pass above me.

Agoura Road travels parallel to the highway and it’s on this road you’ll find another trailhead that heads south into the Santa Monica Mountains.

This is the southern point of the animal crossing and it’s where I meet Sheik Moinuddin. He’s an engineer from Caltrans (District 7) and their project manager for the crossing.

“So when we got the opportunity to build something like this, obviously, we kind of jumped in it, uh, we don’t have the money, but we have the will to do it. And we have the expertise to do it,” said Moinuddin.

This is a first for Caltrans and Moinuddin who will oversee the project.

“It’s a huge over crossing over a very busy freeway. This is the first!” said Moinuddin. “It is just not the first for Caltrans, it is actually first for U.S.”

The basic engineering is pretty amazing. The crossing will be 170 feet wide and 210 feet long. For comparison that would be a bridge about the width of an American football field crossing over 10 lanes of highway.

And because of the geography in the area, it’ll be quite steep on the south side. For every 300 feet there will be a 100 foot drop.

The project will be broken up into two segments. First the bridge over the 101 highway and then the tunnel over Agoura Road.

They’ll start by clearing the vegetation in the area, then they’ll erect the columns. Then the precast bridge will be driven in and placed on top of those columns.

Motorists won’t be affected much, says Moinuddin, as there will only be a couple of late night weekend lane closures.

Moinuddin says he’s hopeful this animal crossing will become a model for the future and he’s happy to be a pioneer of sorts.

“Ah seriously, I’m hoping that this project will show to the world that, yes, this is possible and we can do that,” said Moinuddin.

How the crossing will help wildlife in the region

I take the trail nearby and hike up one of the hills on the south side of the highway. I want to get a bird’s eye view of where the crossing will be.

Seth Riley joins me on the hike. He’s the wildlife branch chief for the Santa Monica Mountains National Recreation area — part of the National Park Service.

This park stretches north from the Pacific Ocean all the way to the 101 in one direction, and from the Hollywood Hills west to just below Port Hueneme in the other. It’s the biggest urban national park in the country, in fact.

“And so all the work that we do to understand and try to preserve wildlife communities and populations is focused on understanding what are the impacts of urbanization and habitat fragmentation on wildlife,” said Riley.

We’ve stopped to talk about halfway up the hill where there’s a great view of the 101 below us. The wind has picked up.

Riley has been with this park for 22 years and they’ve been studying carnivores and mountain lions specifically for about the same amount of time. So they have some pretty decent data about them.

Here’s what they’ve learned.

“The Santa Monica Mountains by themselves are not big enough for a viable population of this species, that’s just really only room for maybe 12 to 15 adults and sub adults, and that’s just not enough demographically or especially genetically in the long run for a viable population,” said Riley.

Male mountain lions have massive home ranges – typically up to 250 square miles. The entire Santa Monica Mountains is 370 square miles so there’s space for only a few dominant males.

The 101, and other highways, have prevented these lions from seeking out new territories and so they’re literally boxed into an area.

“Really important thing we’ve learned about the Mountain Lions is that genetic diversity in the lions in the Santa Monicas is very low, basically the lowest that anyone’s seen,” said Riley.

The genetic diversity is low because of inbreeding. Ok, don’t judge…. But fathers are mating with daughters, grand daughters and even great granddaughters.

Scientists have found sperm quality is very poor with some of the male mountain lions being found to have more than 90 percent abnormal sperm.

Here’s how the animal crossing will help.

“The goal is not for the population to get bigger because it is what it is. The goal is for it to be better connected to the other populations,” said Riley.

But the goal here is that the crossing will benefit all animals in the area – mule deer (prey for mountain lions), bobcats, coyotes, badgers, rabbits, mice, voles, woodrats, horn lizards, tree frogs, snakes, ants, quail and even birds who are strong fliers. Riley says studies have shown that birds use animal crossings to pass over highways in a way they wouldn’t across open highways with cars.

This animal crossing means a lot to Riley because it’s solving a problem. He says often their research doesn’t result in big solutions like this one.

“We learn a lot of things and we convey that information as best we can to other folks. But often it’s not something we can sort of do that much about necessarily and others can and do, which is great. But in this case, we’re really contributing to something that’s actually going to happen,” said Riley.

Conserving the wild space around the crossing has taken decades

And how did we get here? There’s the celebrity puma, the years of fundraising for the animal crossing – and then, before all that, preserving all this incredibly valuable coastal land. It has taken decades to create this wild space.

Rorie Skei is the chief deputy director of the Santa Monica Mountains Conservancy. Skie is in charge of land acquisition.

“It has been putting together these puzzle pieces, so a lot of planning, a lot of working on ‘OK’ this landowner is not a willing seller. How long will it take to get them to be a willing seller?” said Skei.

A couple of miles further south from that windy hilltop with the view of the 101, we’re at King Gillette Ranch. Skei and I sit outside the visitor center where feral parrots chirp in the trees and a herd of mule deer graze nearby. This ranch was a major acquisition years in the making, Skei says.

Open space is sometimes already part of a park system, some of it is donated, but a lot of the time it’s privately owned and very expensive.

“That’s what we found over and over again. Circumstances change. Owners become willing sellers, and then it’s a scramble to try to assemble the money to pay the appraised price,” said Skei.

The biggest and smallest details about the crossing

So this is the part I get excited about… the little details about the animal crossing itself. What’s it going to be like?

To find that out I connect over Zoom with Robert Rock. He’s the principal and COO at Living Habits, and the lead architect on the project. He’s based in Chicago.

Rock speaks with such colorful language when he describes the crossing. He says things like it’ll be a gigantic green roof on steroids. Or..

“That kind of green toupee over the freeway,” said Rock.

He describes the crossing as an ecological stitch that’ll weave together the rich tapestry or wild spaces of the region.

“How can we make this feel like the structure is carrying the mountain across the road? How can we make the structure feel like it really, truly is a visceral connective thread?” said Rock.

He says an important part of this project has been the need to deal with the biggest issues surrounding the crossing.

“How does this sit within the watershed? How does this sit within the Santa Monicas? How does this sit within the state of California and how does it sit within the entire western region?” said Rock.

As well as the tiny tiniest issues that no one will notice

“But then we go all the way down to the microscopic level where we’re talking about building soil ecology, we’re talking about microbial biomass,” said Rock. “Nine out of ten people are not going to even know that we spent all this time thinking about the microbial biomass in the soil and the, you know, the degree to which that links to the carbon sequestration or the minutia of how we design spaces to accommodate the California kingsnake.”

The team will restore the vegetation around the crossing with species native to the area – looking at sage scrub, chaparral and oak woodlands. They’ll build fencing to keep animals away from the highway. They’ll also construct massive walls on the edge of the highway and on the crossing, which will be covered in vegetation that decreases sound.

“To take it a step further we were looking for strategies where we could reduce not just the high and medium frequency sounds that sound walls typically are good at, you know, at buffering for. But we’re also trying to reduce the low frequency sounds,” said Rock.

The height and thickness of the bridge has also been considered to avoid the noise of the cars below.

They’ve also thought about the light. They are looking into lowering street lights on the nearby offramps without affecting safety.

The color palette is taken from the Santa Monica Mountains. Rock says this will help darken the structure at night so you don’t have that reflective glow you sometimes see on concrete bridges.

It’s all to make the bridge as inviting as possible to wildlife.

On the bridge itself they will have plant communities but not giant trees. They don’t want to overload the crossing.

“We’re creating a project nursery for this, where we are going to be growing all the plants that are going to be a part of this construction. And part of that is leveraging, you know, seed bank that the National Park Service has and that we’ll be collecting from the site and from adjacent areas,” said Rock.

They are also collecting native fungi that are linked to these plant communities. The crossing will be an ecosystem of its own.

“We’re going to end up having species that are going to call that crossing home because it’s going to be this piece of habitat that they don’t navigate out of,” said Rock.

When’s it going to be ready?

Construction work starts in April on the Wallis Annenberg Wildlife Crossing, named for the philanthropists who gave a lot of money. There will be a live webcam during construction so you can watch the progress.

Once completed wild spaces will be connected from the Pacific Ocean and Malibu all the way up to the Los Padres National Forest, and many more thousands of square miles of habitat beyond that.

The crossing will be open for business by late 2024, early 2025.

And no, humans will not be allowed on it — just like the name suggests it’s an animal crossing just for them.

For many of the people working on this project it’s been a long process to get to this point. Conserving the land over decades, getting P-22 to inspire grand philanthropy, to design and engineering never seen before. But for the same people they hope this is really just the beginning.

“My primary goal with this is that while I want this to be the best project that it possibly can be. I don’t want it to be the cutting edge for long. I want the next project to be even better,” said Rock.

Further Reading

• Watch a Science Friday video about scientists who built bridges for turtles to cross railroad tracks.