Berlin is the capital and the largest city of Germany as well as one of its 16 constituent states. With a population of approximately 3.7 million, Due to its location in the European Plain, Berlin is influenced by a temperate seasonal climate. Around one-third of the citys area is composed of forests, parks, gardens, rivers, canals and lakes. First documented in the 13th century and situated at the crossing of two important historic trade routes, Berlin became the capital of the Margraviate of Brandenburg, the Kingdom of Prussia, the German Empire, the Weimar Republic and the Third Reich. Berlin in the 1920s was the third largest municipality in the world. After World War II and its subsequent occupation by the victorious countries, the city was divided; East Berlin was declared capital of East Germany, while West Berlin became a de facto West German exclave, surrounded by the Berlin Wall and East German territory. Following German reunification in 1990, Berlin once again became the capital of all-Germany. Berlin is a world city of culture, politics, media and science. Berlin serves as a continental hub for air and rail traffic and has a highly complex public transportation network. The metropolis is a popular tourist destination. Significant industries also include IT, pharmaceuticals, biomedical engineering, clean tech, biotechnology, construction and electronics. Modern Berlin is home to world renowned universities, orchestras, museums, entertainment venues and is host to many sporting events.