Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally divided from Asia to its east by the water divide of the Ural Mountains, the Ural River, the Caspian Sea, and by the Caucasus Mountains to the southeast. Europe is washed upon to the north by the Arctic Ocean and other bodies of water, to the west by the Atlantic Ocean, to the south by the Mediterranean Sea, and to the southeast by the Black Sea and the waterways connecting it to the Mediterranean. Yet, the borders for Europe—a concept dating back to classical antiquity—are somewhat arbitrary, as the term continent can refer to a cultural and political distinction or a physiographic one. This article is primarily about the first two aspects, although it references Europe's geography. Europe is the world's second-smallest continent by surface area, covering about 10,180,000 square kilometres (3,930,000 sq mi) or 2% of the Earth's surface and about 6.8% of its land area. Of Europe's approximately 50 states, <a href="/call/russia/">Russia</a> is the largest by both area and population, while the <a href="/call/vatican-city/">Vatican City</a> is the smallest. Europe is the third most populous continent after Asia and Africa, with a population of 731 million or about 11% of the world's population; however, according to the United Nations (medium estimate), Europe's share may fall to about 7% in 2050.