In winter, seawater freezes and forms a crust of ice called "pack ice". The area of floating ice, which makes up much of the ice cap in the Arctic Ocean, expands during winter to cover about 5% of the northern oceans and 8% of the southern oceans.

Pack ice consists of ice that formed both at sea and as fast ice (locked to the shore). It can be very flat (because the ocean is flat), but it is usually covered with very rough areas caused by the movement of sheets of ice against one other. These pressure ridges can increase the thickness of the ice from just a few inches or centimeters to tens of meters (many feet) thick.

Although pack ice moves with ocean currents and wind, it is not free-floating like ice-floes, and it is not always continuous. At times it can be very broken, with leads (cracks of open water) opening up without warning. The leads then refreeze, adding new ice throughout the winter.

For many years the covering of pack ice was seen mainly as a danger and an obstacle to trade. We now know that the sea ice not only dominates the polar regions and is important to global ocean circulation and climate patterns, but that the ice itself is now endangered because of global warming.