In winter, a quixotic carapace of drifting sea ice blankets millions of square miles of the polar ocean. Iñupiaq Eskimo call the permanent part of the polar pack, consisting of both multiyear and annual pack ice, aakanga siku, “mother ice.” The English-language term is meant to include, as well, the extensive ﬁelds of younger, mobile ice-of-theyear, the seasonal or annual ice that may stretch much farther to the south as winter settles in. During storms, pack ice cries out: grinding, screeching. Winds, tides, and currents mangle floes into pressure ridges or rip them apart to form temporary open water leads. When Ernest Shackleton’s ship Endurance became frozen in Antarctic pack in January 1915, the dynamic nature of the ice was made manifest in a ship that was first cemented, then splintered, and finally sunk in place, all while the pack traveled 670 miles in ten months, shuddering and groaning incessantly. In his famous poem, Coleridge depicts the fearful aspects of pack ice encountered by the Ancient Mariner: “The ice was here, the ice was there/The ice was all around:/It cracked and growled, and roared and howled,/Like noises in a swound!” Ishmael, Moby-Dick’s narrator, calls pack ice “a boundless church-yard grinning . . . with its lean ice monuments and splintered crosses.” Eskimos in Arctic Alaska hold quite a different view. For these native people, annual ice forming over the continental shelf equals life. In summer, melting pack ice releases freshwater and nutrients, helping spin a food web: algae, flagellates, zooplankton, ﬁshes, mammals. Polar bears and many species of seal and whale are entirely dependent on pack ice for survival. When the pack drifts far to the north, Eskimo hunters scan the seaward horizon expectantly for its reflection in the sky—ice blink—a brightness in the clouds. In recent years, summer pack ice, reduced by global warming, remains farther offshore and less accessible to hunters. In this century, if current projections of continuous climate warming are correct, the shrinking ice pack will transform the Northwest Passage from occasional adventure to commonplace reality.