The Arctic is the northernmost polar region of Earth, centred on the Arctic Ocean. Unlike Antarctica, which is an isolated continent covered in ice, the Arctic is a frozen ocean almost completely ringed by the landmasses of eight sovereign nations: Canada, Greenland (Denmark), Iceland, Norway, Sweden, Finland, Russia, and the United States (Alaska). It has extreme light cycles, switching from continuous summer Midnight Sun to total winter Polar Night, and supports uniquely adapted marine mammals, apex land predators, and thousands of years of indigenous human habitation.