Europe ( or ) is a continent that comprises the westernmost peninsula of Eurasia. It is generally divided from Asia by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting the Black and Aegean Seas. Europe is bordered by the Arctic Ocean to the north, the Atlantic Ocean to the west, the Mediterranean Sea to the south, and the Black Sea and connected waterways to the southeast. Yet the borders of Europe—a concept dating back to classical antiquity—are arbitrary, as the primarily physiographic term “continent” also incorporates cultural and political elements. Europe is the world’s second-smallest continent by surface area, covering about or 2% of the Earth’s surface and about 6.8% of its land area. Of Europe’s approximately 50 countries, Russia is by far the largest by both area and population, taking up 40% of the continent (although the country has territory in both Europe and Asia), while Vatican City is the smallest. Europe is the third-most populous continent after Asia and Africa, with a population of 739–743 million or about 11% of the world’s population. The most commonly used currency is the euro. Europe, in particular ancient Greece, is the birthplace of Western culture. It played a predominant role in global affairs from the 15th century onwards, especially after the beginning of colonialism. Between the 16th and 20th centuries, European nations controlled at various times the Americas, most of Africa, Oceania, and the overwhelming majority of Asia. The Industrial Revolution, which began in Great Britain around the end of the 18th century, gave rise to radical economic, cultural, and social change in Western Europe, and eventually the wider world. Demographic growth meant that, by 1900, Europe’s share of the world’s population was 25%. Both world wars were largely focused upon Europe, greatly contributing to a decline in Western European dominance in world affairs by the mid-20th century as the United States and Soviet Union took prominence. During the Cold War, Europe was divided along the Iron Curtain between NATO in the west and the Warsaw Pact in the east. European integration led to the formation of the Council of Europe and the European Union in Western Europe, both of which have been expanding eastward since the revolutions of 1989 and the fall of the Soviet Union in 1991. The European Union nowadays has growing influence over its member countries. Many European countries are members of the Schengen Area, which abolishes border and immigration controls among its members.