When the 101 freeway was built through the Santa Monica Mountains nearly 100 years ago, it trapped wildlife — lizards, birds and famously, mountain lions — on either side of it.  That’s led to inbreeding and even the threat of extinction.

“We’ve perpetuated so much harm on life on this planet. I mean, we humans have a lot to answer for,” says Beth Pratt, California director for the National Wildlife Federation. 

For more than a decade, she’s advocated for Caltrans to build a bridge to help animals like LA’s famed mountain lion P22 to expand their geographic territory.

Now that long-dreamed-of bridge is taking shape.

The Wallis Annenberg Crossing will be the largest animal bridge in the world, offering a safe way to traverse the Santa Monica Mountains over the interstate below. 

Caltrans is installing more than 80 large concrete supports, called girders, to shape the bridge so it will hold animals that cross it as well as an entire natural ecosystem on top. 

That construction work is forcing the agency to close a section of the busy road every weeknight through late May. 

Pratt describes the bridge as “righting a great wrong.”

“This freeway wreaked devastation on wildlife here,” she says. “And we’re going to fix this one.”

The crossing design team has been researching and experimenting for years to create a bridge that all types of wildlife will actually use, despite the 400,000 cars that drive the 101 each day. 

The first problem? Noise. Mountain lions, foxes, and lizards have more sensitive ears than humans and try to avoid traffic noise. 

“We’ve got ambient sounds coming off of the freeway at between 80 and 90 decibels,” says Robert Rock, president and CEO of Rock Design Associates and lead designer of the crossing. “How that impacts a natural habitat is pretty significant across a whole suite of different species.”

Rock says they’re using two types of walls to block around 20 decibels of noise from the freeway – one made of compacted soil and one made of rocks. They are also planning on growing wild grape vines over the bridge walls to absorb more sound. 

Second, there’s the brightness problem. Most animals are active at night, when traffic is quieter. But they’re also scared off by street lights and car headlights. 

Caltrans project manager Sheik Moinuddin says they’ll use specially tailored lights on the freeway around this project. 

“We’re not going to have any lights on top,” he says. “And even the soft lighting underneath the bridge, we’re shielding it in a way that it focuses right on the pavement rather than dispersing all over the place.”

The 6,000 tons of concrete that have been poured to create the crossing was also made with a specific pigment that reduces its surface reflectivity. That means less light from car headlights will bounce off the concrete and illuminate the areas around the crossing. 

Lastly, the design team wanted the crossing itself to look and feel as natural as possible, though they’re creating that habitat out of thin air. 

“I’ve referred to it as a green roof on steroids,” Rock says. “It’s a giant green toupee on top of a big chunk of concrete.”

They are taking a slice of the native habitat – starting from the plants above ground all the way down to the microorganisms in the soil – and dropping around 175 feet of that on top of the crossing. 

A team spent years collecting millions of seeds, fungi, and microorganisms, all from within a five-mile radius of the crossing. They’re already growing the plants – including local sages, buckwheat, milkweeds, and grasses – in a nearby nursery that will one day be planted on top.

Cultivating native fungi and microorganisms will also prime the soil to help the plants absorb more water (making them more resilient during wildfires) and connect the habitat below ground. 

“This is about restoring the entire ecosystem back across the freeway,” Rock says.

The design team will use rainwater that filters off the crossing to reconnect a creek that was separated by the 101 freeway, which will help direct animals to the crossing. 

“Many of these species navigate based on where the natural ridge lines or natural draws are at,” Rock says. “We’re trying to restore as many of those natural ridge lines and draws as we can.”

The hope is that this crossing will be a catalyst and an inspiration for more animal bridges in California and the world.

“If LA can do this, if we can put a wildlife crossing over one of the busiest freeways in the world, then nobody has any excuse,” Pratt says.

For the first time last year, Caltrans flagged 43 locations in California that need a wildlife crossing – 11 are in Southern California.

But they’re expensive. The agency needs more than $900 million over the next decade to make those a reality. As of last year, only $69 million was in the pipeline.

The Wallis Annenberg Wildlife Crossing is set to be completed by late 2025 or 2026. Follow construction progress via this livestream.