The Beaverhead impact structure is a meteorite crater extending across parts of Idaho and Montana in the Beaverhead region of southwestern Montana. Measuring about 60 kilometers, or 37 miles, across, it is the second-largest known impact structure in the United States and one of the larger ones on Earth. The impact is thought to have occurred about 600 million years ago in the Neoproterozoic, and its great age has left little of the original crater visible. Among the clearest remaining signs are shatter cones around its perimeter, rock features formed by the intense pressures of the strike. Its importance rests in both its immense size and extraordinary antiquity.