Crews work along Last Chance Grade, where ongoing landslide repairs fight to keep Highway 101 open—California’s only coastal link to Crescent City.

A long-awaited fix is finally taking shape for one of the most hazardous and landslide-prone stretches of California’s Highway 101: a massive tunnel project that would become the longest in the state’s history.

The trouble spot lies deep in the so-called Redwood Curtain — the remote, forested region of California’s far North Coast, where Highway 101 clings to steep cliffs above the Pacific. It’s a rugged, isolated stretch of road, more than five hours north of San Francisco and nearly as far from Portland. When it fails, entire communities like Crescent City and the hamlet of Klamath are cut off.

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This 3-mile stretch of highway sits on a tangled web of overlapping landslides, Caltrans spokesperson Myles Cochrane told SFGATE in an email. Caltrans faces a daunting task, Cochrane explained: keeping the road open and safe while also working toward a permanent fix. That has meant years of slope monitoring, field studies and emergency repairs — all while planning a massive tunnel to bypass the unstable zone entirely.

The California Transportation Commission recently allocated $40 million to support what Cochrane described as “part of the design phase of the Last Chance Grade Project along U.S. Highway 101 south of Crescent City in Del Norte County.” The funding will go toward designing a 6,000-foot tunnel that would avoid the notoriously unstable cliffs at Last Chance Grade — a section so prone to landslides it’s become infamous among travelers and engineers alike.

Currently, California’s longest highway tunnel is the Wawona Tunnel in........

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