Berlin ( bur-LIN, German: [bɛʁˈliːn] ) is the capital and largest city of Germany, both by area and by population. Its more than 3.85 million inhabitants make it the European Union's most populous city, as measured by population within city limits. The city is also one of the states of Germany, and is the third smallest state in the country in terms of area. Berlin is surrounded by the state of Brandenburg, and Brandenburg's capital Potsdam is nearby. The urban area of Berlin has a population of over 4.5 million and is therefore the most populous urban area in Germany. The Berlin-Brandenburg capital region has around 6.2 million inhabitants and is Germany's second-largest metropolitan region after the Rhine-Ruhr region, and the sixth-biggest metropolitan region by GDP in the European Union. Berlin was built along the banks of the Spree river, which flows into the Havel in the western borough of Spandau. The city incorporates lakes in the western and southeastern boroughs, the largest of which is Müggelsee. About one-third of the city's area is composed of forests, parks and gardens, rivers, canals, and lakes. First documented in the 13th century and at the crossing of two important historic trade routes, Berlin was designated the capital of the Margraviate of Brandenburg (1417–1701), Kingdom of Prussia (1701–1918), German Empire (1871–1918), Weimar Republic (1919–1933), and Nazi Germany (1933–1945). Berlin served as a scientific, artistic, and philosophical hub during the Age of Enlightenment, Neoclassicism, and the German revolutions of 1848–1849. During the Gründerzeit, an industrialization-induced economic boom triggered a rapid population increase in Berlin. 1920s Berlin was the third-largest city in the world by population. After World War II and following Berlin's occupation, the city was split into West Berlin and East Berlin, divided by the Berlin Wall. East Berlin was declared the capital of East Germany, while Bonn became the West German capital. Following German reunification in 1990, Berlin once again became the capital of all of Germany. Due to its geographic location and history, Berlin has been called "the heart of Europe". The economy of Berlin is based on high tech and the service sector, encompassing a diverse range of creative industries, startup companies, research facilities, and media corporations. Berlin serves as a continental hub for air and rail traffic and has a complex public transportation network. Tourism in Berlin makes the city a popular global destination. Significant industries include information technology, the healthcare industry, biomedical engineering, biotechnology, the automotive industry, and electronics. Berlin is home to several universities such as the Humboldt University of Berlin, Technische Universität Berlin, the Berlin University of the Arts and the Free University of Berlin. The Berlin Zoological Garden is the most visited zoo in Europe. Babelsberg Studio is the world's first large-scale movie studio complex and the list of films set in Berlin is long. Berlin is also home to three World Heritage Sites: Museum Island, the Palaces and Parks of Potsdam and Berlin, and the Berlin Modernism Housing Estates. Other landmarks include the Brandenburg Gate, the Reichstag building, Potsdamer Platz, the Memorial to the Murdered Jews of Europe, and the Berlin Wall Memorial. Berlin has numerous museums, galleries, and libraries.