The Arctic ice pack is the ice cover of the Arctic Ocean and its vicinity. The Arctic ice pack undergoes a regular seasonal cycle in which ice melts in spring and summer, reaches a minimum around mid-September, then increases during fall and winter. Summer ice cover in the Arctic is about 50% of winter cover. Some of the ice survives from one year to the next. Currently 28% of Arctic basin sea ice is multi-year ice, thicker than seasonal ice: up to 3–4 meters (9.8–13.1 ft) thick over large areas, with ridges up to 20 meters (65.6 ft) thick. As well as the regular seasonal cycle there has been an underlying trend of declining sea ice in the Arctic in recent decades.