Once snow is about 6 inches deep, it insulates the ground to a steady 32°F, creating the subnivean zone where mice, voles and shrews stay active all winter. (156 characters, limit 130-160) Red foxes pinpoint them by sound and dive headfirst through the snow, while great gray owls hover, punch through crusted snow and snatch voles from up to 18 inches down. Meet the hidden winter world beneath the snowpack, and the predators that evolved to raid it.