The fissure was first spotted in October on Rattlesnake Ridge in south central Washington State, overlooking Interstate 82 and the Yakima River. Since then, a 20-acre chunk of mountainside — roughly four million cubic yards of rock, enough to fill 25 football stadiums to the top of the bleachers, eight stories up — has been sliding downhill. Geologists can measure its current speed — about two and a half inches a day — but they cannot say for certain when, or if, it might accelerate into a catastrophe. And they are powerless to stop it. “The mountain is moving, and...