Scientists say they've discovered a massive landslide in an uninhabited area of eastern Alaska that's the largest detected in North America since the 1980 eruption of Mount St. Helens.

The Oct 17 landslide unleashed 200 million tons of rock down the Taan Fjord valley onto Tyndall Glacier in Icy Bay, according to Columbia University's Lamont-Doherty Earth Observatory.

No one witnessed it, but scientists picked up its "seismic signature" with a method of reading patterns they've been honing for six years.