With an area of , Romania is the second biggest country in southeastern Europe. It is the twelfth-largest in Europe. A large part of Romania's border with Serbia and Bulgaria is made by the Danube. The Danube is joined by the Prut River. The Prut River makes the border with the Republic of Moldova. The Danube then flows into the area of the Black Sea inside of Romania, which forms the Danube Delta. The Danube Delta is the second biggest one of the best kept deltas in Europe. It is also a biosphere reserve and a biodiversity World Heritage Site. Some of the other major rivers in Romania are the Siret, which runs from the north to the south of Moldavia; the Olt, which runs from the Carpathian Mountains to Oltenia; and the Mureş, which runs through Transylvania from the east to the west.