An ice sheet is a large mass of glacial ice that covers land, typically in polar regions. It forms through the accumulation and compaction of snow over thousands or millions of years, creating a thick layer of solid ice that can be several thousand meters deep. Ice sheets are different from sea ice, which forms on top of oceans and is much thinner. The two largest ice sheets in the world are located in Greenland and Antarctica, covering vast areas and exerting significant influence over global climate patterns through their effects on albedo (reflectivity) and meltwater discharge.