California taxpayers are staring down a multibillion-dollar price tag to rescue one of the state’s most unstable highways, a collapsing coastal stretch that is quite literally sliding into the ocean.

The California Department of Transportation is now advancing a plan to bore a 1.1-mile tunnel through redwood forest to bypass the deteriorating “Last Chance Grade” section of US Highway 101, with the project estimated to cost $2.5 billion.

That staggering figure comes after more than a decade of study, outreach and planning, capped by a 712-page environmental impact report that cost $55 million and was released in late May.

Next up, Caltrans is expected to ask the California Transportation Commission for $225 million later this summer just to fund the tunnel’s design phase, bringing in international experts in seismic tunnel construction.

The proposed fix targets a three-mile stretch of highway in Del Norte County that clings to fog-covered cliffs between ancient redwoods and the Pacific Ocean.

The route is the only realistic road connection between Crescent City, a tsunami-prone town of about 6,000 residents, and Humboldt County and the broader region.

When the road shuts down, drivers are forced into a brutal 449-mile, eight-hour detour through Redding and southern Oregon, unless they attempt steep, unpaved logging roads.

The instability isn’t new.

The corridor began life as a wagon trail in 1894 and was rebuilt in the 1930s, despite early warnings from engineers that constant land movement would make maintenance expensive.

Today, the highway sits on four active landslides and has become one of California’s most problematic routes.

In the past, it was reduced to one-way traffic for nine straight years, briefly reopened in October 2023, and has since faced repeated restrictions.

The ground beneath it has shifted dramatically, up to 40 feet horizontally and 30 feet vertically since the 1930s, with some sections now moving several feet each year toward the Pacific.

In 1972, a collapse in the pre-dawn hours sent a vehicle off the cliff, killing a married couple.

The state’s solution is an eastward tunnel that would bypass the most dangerous terrain.

At 6,000 feet long, it would become the longest highway tunnel in California, surpassing the 4,233-foot Wawona Tunnel in Yosemite National Park.

But the fix carries environmental consequences as well.

The route cuts through Redwood National and State Parks, a UNESCO World Heritage Site, and would require the removal of 16 old-growth redwood trees wider than four feet, along with additional trees, according to the environmental impact report.

Jaime Matteoli, the Caltrans project manager for Last Chance Grade, called the effort a necessary investment despite the scale of the challenge.

“It’s a proud moment,” Jaime Matteoli, the Last Chance Grade project manager for Caltrans told The Los Angeles Times. “It’s a huge quality-of-life issue for people, feeling safe on that road. It’s universally recognized that this project is needed.”

If funding moves forward, construction could begin around 2031, with the tunnel potentially opening by 2039.