Sofia's development as a significant settlement owes much to its central position in the Balkans. It is situated in western Bulgaria, at the northern foot of the Vitosha mountain, in the Sofia Valley that is surrounded by mountains on all sides. The valley is the largest in the country with territory of 1,186 square kilometres (458 sq mi) and average altitude of 550 metres (1,800 ft). Three mountain passes lead to the city, which have been key roads since antiquity, connecting the Adriatic Sea and Central Europe with the Black and Aegean Seas. A number of low rivers cross the city, including the Vladaiska and the Perlovska. The Iskar River in its upper course flows near eastern Sofia. The city is known for its numerous mineral and thermal springs. Artificial and dam lakes were built in the last century.