In glaciology, an ice sheet, also known as a continental glacier, is a mass of glacial ice that covers surrounding terrain and is greater than 50,000 km2 (19,000 sq mi). The two currently existing ice sheets are the Antarctic ice sheet and the Greenland ice sheet. Ice sheets are the largest glacial bodies on Earth, distinguished from smaller ice caps or alpine glaciers. Ice sheets can have multiple ice domes, the topographic highs from which ice flows outwards, and are typically drained by ice streams and outlet glaciers around their periphery.