As the historic core of Russia, Moscow serves as the home of numerous Russian artists, scientists, and sports figures due to the presence of its various museums, academic and political institutions, and theatres. The city is home to several UNESCO World Heritage Sites, and is well known for its display of Russian architecture, particularly its historic Red Square, and buildings such as the Saint Basil's Cathedral and the Moscow Kremlin, of which the latter serves as the seat of power of the Government of Russia. Moscow is home to many Russian companies in numerous industries and is served by a comprehensive transit network, which includes four international airports, nine railway terminals, a tram system, a monorail system, and most notably the Moscow Metro, the busiest metro system in Europe, and one of the largest rapid transit systems in the world. The city has over 40 percent of its territory covered by greenery, making it one of the greenest cities in Europe and the world.