Finland is a Northern European nation bordering Sweden, Norway and Russia. Its capital, Helsinki, occupies a peninsula and surrounding islands in the Baltic Sea. Helsinki is home to the 18th-century sea fortress Suomenlinna, the fashionable Design District and diverse museums. The Northern Lights can be seen from the country's Arctic Lapland province, a vast wilderness with national parks and ski resorts. In the earliest historical sources from the 12th and 13th centuries, the term Finland refers to the coastal region around Turku from Perniö to Uusikaupunki. This region later became known as Finland Proper in distinction from the country name Finland. Finland became a common name for the whole country in a centuries long process that started when the Catholic Church established missionary diocese in Nousiainen in the northern part of the province of Suomi possibly sometime in the 12th century. Winters in southern Finland (when mean daily temperature remains below 0 °C or 32 °F) are usually about 100 days long, and in the inland the snow typically covers the land from about late November to April, and on the coastal areas such as Helsinki, snow often covers the land from late December to late March. In northern Finland, particularly in Lapland, the winters are long and cold, while the summers are relatively warm but short. A quarter of Finland's territory lies within the Arctic Circle and the midnight sun can be experienced for more days the farther north one travels. At Finland's northernmost point, the sun does not set for 73 consecutive days during summer, and does not rise at all for 51 days during winter. The Finnish landscape is covered with thick pine forests and rolling hills, and complemented with a labyrinth of lakes and inlets. Much of Finland is pristine and virgin as it contains 37 national parks from the Southern shores of the Gulf of Finland to the high fells of Lapland. Finland also has urbanised regions with many cultural events and activities. Commercial cruises between major coastal and port cities in the Baltic region, including Helsinki, Turku, Tallinn, Stockholm, and Travemünde, play a significant role in the local tourism industry.