LOS ANGELES — Monday was momentous for the Wallis Annenberg Wildlife Crossing although it still looked like a bridge to nowhere from the 101 Freeway, where more than 300,000 vehicles stream endlessly every day.

Nearly three years after the project began, the critical milestone was visible only to the government officials, scientists and longtime supporters who climbed to the top: soil.

And not just any soil. Over the next few days they'll be adding 6,000 cubic yards of specially manufactured soil to cover the crossing, a mix of sand, silt and clay inoculated with a bit of compost and hyperlocal mycorrhizal fungi, carefully designed and tested to mimic the biological makeup of native soils around the site.

Adding soil is a big deal because it means we're nearing the end of Stage 1, when the top is seeded and then planted with native shrubs and perennials, work that should be completed this summer, said Robert Rock, chief executive of Chicago-based Rock Design Associates and the landscape architect overseeing the project.

But we're still a ways from completion, now scheduled for the end of 2026, about a year later than initially planned due to delays from the heavy rains in 2022 and 2023.

Stage 2, the final phase, will connect the structure to the hills at the north and south so that wildlife, like L.A.'s famous, ill-fated cougar, P-22, can use it. That stage will start with burying utility lines along a section of Agoura Road, south of the crossing, and moving water lines for recycled water used for irrigation.

Late in 2025 or early 2026, Agoura Road will be closed for a few months so the roadway can be covered with a tunnel and lots of soil collected from the Malibu Lake area. The goal is to create natural slopes off the crossing — an additional 12 acres of space that will be planted with native shrubs, perennials, grasses and trees to make the crossing blend as much as possible with the surrounding hills.

"Basically they're restoring the mountain to what it once was, so wildlife won't even know they're crossing the freeway," said Jeremy Wolf, mayor pro tem of Agoura Hills, who was among several government officials, scientists and longtime advocates who came to celebrate the soil's arrival on Monday.

"Humans created this problem," Wolf said in an interview. "We've created islands of habitat fragmentation with our roadways and housing encroaching deeper and deeper into the wild urban interface, and now we're fixing this problem by using human ingenuity for good purposes."

There has been plenty of ingenuity and innovation along the way and lots of attention to detail. For instance, even before construction began in 2022, the project built a nursery and sent horticulturists combing the hills around the site to collect about 1 million seeds from around 30 native plant species. Those seeds were planted and have been nurtured into 1-gallon pots.

Mycologists (biologists who study fungi) collected beneficial mycorrhizal fungi and other microbes from the area to inoculate the soil used to grow the plants, and cover the top of the structure.

They also tested what native seeds best interacted with that fungi, and chose four to sow as a cover crop before adding the plants to help "jump-start" beneficial fungi growth in the soil, which helps plants access the nutrients and water they need to thrive. Those seeds will be sown later this month, Rock said: golden yarrow (Eriophyllum confertiflorum), California poppy (Eschscholzia californica), giant wildrye (Elymus condensatus) and Santa Barbara milk vetch (Astragalus trichopodus), which supports more than 20 butterflies, including the endangered Palos Verdes blue butterfly.

It took nearly a year's worth of prep to get the top ready for soil by building thick walls 12 feet tall to minimize traffic noise and visibility and adding a special drainage system to ensure water doesn't accumulate on the structure, a rubber waterproof lining and then 9 to 30 inches of a lightweight aggregate known as expanded clay shale to enhance drainage, because most native plants don't like wet feet.

Meanwhile, the soil was mixed and "matured" at a site in Lopez Canyon north of Pacoima, where it has been analyzed and adjusted to make sure the pH levels and chemical compounds are balanced. Rock said their final mix had slightly elevated chlorine levels, so they had to water it deeply to help dilute and flush out that excess chemical compound.

On Monday, a small army of supporters and media climbed to the top of the structure to witness the spreading of the soil. Visitors included former state Sen. and Assemblymember Fran Pavley, Agoura Hills' first mayor after the city incorporated in 1982, who fought to keep wild areas open on both sides of the 101 Freeway for some kind of wildlife crossing; wildlife biologists Miguel Ordeñana (who discovered P-22), Jeff Sikich and Seth Riley, who have long tracked and researched mountain lions and other wildlife in the Santa Monica Mountains, and Beth Pratt, the California regional executive director for the National Wildlife Federation, which helped raise the $92 million in state and private funds to build the corridor.

Shortly before 8 a.m. a long, narrow conveyor device started pulling the soil from trucks at the north side of the freeway up 60 feet to the top, where it was spread around the structure with compact track loaders, essentially small earth movers that use tracks instead of wheels to minimize soil compaction.

The top of the structure is 174 feet wide — wider than a football field. If the soil were spread uniformly it would be about 18 inches deep, Rock said, but the goal is to mimic the adjacent hills, so the soil is being contoured 1 to 4 feet deep, with a few pockets of small sandstone boulders. In the next month or so a few large volcanic rocks will also be craned up to the top, to match the geology of the region, which includes a small streak of volcanic rock.

After the seeds have a chance to grow and energize the soil, the mature native shrubs and perennials will be planted, probably in mid-May, Rock said. Those plants, grown from locally collected seed, include black sage, white sage and purple sage, California buckwheat, long-stem buckwheat and ashy leaf buckwheat, wild grape; narrow-leaf milkweed, California bush sunflower; deerweed; showy penstemon, toyon, laurel sumac and ceanothus.

They'll also be keeping a sharp eye open for invasives that could crowd out the newly planted native plants, aggressive plants such as black mustard, which is already sprouting lushly along the southern border of the corridor. Black mustard grows quickly and will soon be sending out seeds that could easily invade the pristine soil on the structure.

But in this matter, the builders of the world's largest wildlife corridor are in the same boat as gardeners everywhere. "We'll just have to weed it until the [native] plants get established," he said. "The sad reality is there is very little else we can do."

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