Viewed from space, the Earth appears as a "blue planet"; its surface is covered to a large extent by water. The regions around the South and North Pole are particularly striking. The Antarctic is almost completely covered by an ice shield up to several thousand meters (several miles) thick, and in the Arctic, large areas around the North Pole and Greenland are covered by ice. The huge ice shelf areas in Antarctica are particularly impressive, but there is also ice in other parts of the world: even at the equator high peaks are glaciated. In the mountains, glaciers represent an essential freshwater reserve that feeds many rivers. Water is a very special liquid, as it at its highest density and smallest volume at 4 °C (39 °C). This property, described by physicists as the anomaly of water, means that ice is lighter than water, which is why icebergs float on water and lakes freeze on the surface. Ice forms a hostile environment, but humans and animals have adapted to life on the ice. Indigenous peoples such as the Inuit in the Arctic have developed survival strategies over thousands of years. However, the traditional way of life of Arctic inhabitants is changing, and they are being forced to adapt. Some species such as the polar bear are threatened with extinction. This is due to climate change, which is causing temperatures to rise worldwide, particularly rapidly in the Arctic and Antarctic. As a result, glaciers are melting ever more quickly. Ice, which has always fascinated people with its bizarre beauty, is disappearing, and the effects on the environment and the economy are serious.