Drift ice is any sea ice other than fast ice, the latter being attached ("fastened") to the shoreline or other fixed objects (shoals, grounded icebergs, etc.). Drift ice is carried along by winds and sea currents, hence its name. When drift ice is driven together into a large single mass (>70 overage), it is called pack ice. Wind and currents can pile up that ice to form ridges up to several metres in height. These represent a challenge for icebreakers and offshore structures operating in cold oceans and seas.