Single ice cap, Greenland .
Covering about 80% of the island of Greenland, it is the largest ice mass in the Northern Hemisphere, second only to the Antarctic. It extends 1,570 mi (2,530 km) north to south, has a maximum width of 680 mi (1,094 km) near its northern margin, and an average thickness of about 5,000 ft (1,500 m). The ice sheet rises to two domes; the northern dome, reaching more than 10,000 ft (3,000 m), is its thickest and coldest point. In volume it contains 12% of the world's glacial ice. If it melted, the sea level would rise 20 ft (6 m).