The state of Bulgaria [bʌlˈgɛɹiə] (Bulgarian: България, transliterated: Balgaria, pronounced IPA: [bəlˈgarija]), international transliteration Bălgarija, officially the Republic of Bulgaria (Република България, Republika Balgaria, pronounced IPA: [rɛˈpubliˌka bəlˈgarija]) forms part of the Balkans in south-eastern Europe. It borders five other countries: Romania to the north (mostly along the River Danube), Serbia and the Republic of Macedonia to the west, and Greece and Turkey to the south. The Black Sea defines the extent of the country to the east. Bulgaria comprises the classical regions of Moesia, Thrace, and Macedonia. Old European culture within the territory of present-day Bulgaria started to produce golden artifacts by the fifth millennium BC. Currently Bulgaria functions as a parliamentary democracy under a unitary constitutional republic. A member of the European Union since 2007 and of NATO since 2004, it has a population of approximately 7.6 million, with Sofia as its capital and largest city. Geographically and in terms of climate, Bulgaria features notable diversity with the landscape ranging from the Alpine snow-capped peaks in Rila, Pirin and the Balkan Mountains to the mild and sunny Black Sea coast; from the typically continental Danubian Plain (ancient Moesia) in the north to the strong Mediterranean climatic influence in the valleys of Macedonia and in the lowlands in the southernmost parts of Thrace. The Balkan Peninsula derives its name from the Balkan or Stara Planina mountain-range, which runs through the centre of Bulgaria and extends into eastern Serbia.