The North Pole is the northernmost point on Earth, lying diametrically opposite the South Pole. It defines geodetic latitude 90° North, as well as the direction of True North. At the North Pole all directions point south; all lines of longitude converge there, so its longitude can be defined as any degree value. While the South Pole lies on a continental land mass, the North Pole is located in the middle of the Arctic Ocean amidst waters that are almost permanently covered with constantly shifting sea ice. This makes it impractical to construct a permanent station at the North Pole (unlike the South Pole). In recent years, journeys to the North Pole by air (landing by helicopter or on a runway prepared on the ice) or by icebreaker have become relatively common event, and are available to small groups of tourists through adventure holiday companies. The most exciting way is to reach the Pole by ski crossing last degree (111 km).