It was big, potentially bad, and fell at the right time -- sometime in the night when there was no snowmobile or snowcoach traffic coming into Yellowstone National Park from the West Entrance.
The blockish boulder, equal in size to a large SUV, but certainly much heavier, came down sometime in the night late Wednesday or early Thursday about nine miles inside the park from West Yellowstone, Montana.
Park officials aren't exactly sure why it fell. Some seismic activity had been noted at a nearby station, Yellowstone spokesman Dan Hottle says. But, he adds, there is some question as to whether a seismic event triggered the boulder fall, or whether the crashing down of the boulder was recorded seismically.
Park crews have pushed the boulder off the road, and will now determine the best way to break up the rock and remove it.