A classic Hollywood movie scene is the panoramic shot of a lone horseback rider, out on the grassy rolling plain. The horse stops, fidgeting. At first, all the horseman can hear is the sound of the gentle breeze under a stark blue sky. Then, a sub-aural rumble fills the air. Growing louder, the horseman stands up in the saddle and squints in the direction of the sound. We, in the audience, look, too — and leap out of our skins when dozens of bad guys come roaring over the nearest hillock, right at our hero.