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.