Sofia is located at about 550 m asl, in the foothill of the northern slope of the Vitosha, in the Sofia plain enclosed by mountains on all sides. As early as Antiquity, the five mountain passes leading to the city (Iskar, Vladaya Dragoman, Petrohan and Botevgrad) and the roads passing there connected the Adriatic and Central Europe with the Black and Aegean Seas and the Middle East. Thanks to its central position in the Balkans, Sofia is a large and flourishing city to this day. Since the remote past the city is known for its numerous hot and cold mineral springs (15 fields with a total output of mineral water of 130 l/sec). The Iskar, the longest river running entirely in the territory of Bulgaria, traverses the city.