The Rodopi occupies the area between the Thracian Plain and the Greek border. This range is commonly described as including the Rila mountain range south of Sophia and the Pirin range in the southwestern corner of the country. As such, the Rodopi is the most outstanding topographic feature, not only of the country, but also of the entire Balkan Peninsula. The Rila contains Mount Musala--called Mount Stalin for a few years--whose 9,500-foot peak is the highest in the Balkans. About a dozen other peaks in the Rila are over 9,000 feet. They feature a few bare rocks and remote lakes above the tree line, but the lower peaks are covered with Alpine meadows, and the general aspect of the range is one of green beauty. The Vitosha range is an outlier of the Rila. A symmetrical, 7,500-foot high, isolated peak in the range is a landmark on the outskirts of Sofia. Snow covers its conical summit most of the year, and its steep sides are forested. The Pirin is characterized by rocky peaks and stony slopes. An impression of the landscape is provided by a local legend, which says that when the earth was being created God was flying over the peninsula with a bag of huge boulders. The rocks were too heavy for the bag, and it broke over southwestern Bulgaria. Some Bulgarian geographers refer to the western Rodopi and the Pirin as the Thracian-Macedonian massif. In this context, the Rodopi includes only the mountains south of the Maritsa River basin. There is some basis for such a division. The Rila is largely volcanic in origin. The Pirin was formed at a different time by fracturing of the earth's crust. The uplands east of the Maritsa River are not of the same stature as the major ranges. Sizable areas in the western and central Stara Planina and smaller areas in the Pirin and in Dobrudzha have extensive layers of limestone. There are some 2,000 caves in these deposits. The public has become more interested in the caves during the past three or four decades, but only about 400 of them have been completely explored and charted. To the east of the higher Rodopi and east of the Maritsa River are the Sakar and Strandzha mountains. They extend the length of the Rodopi along the Turkish border to the Black Sea but are themselves comparatively insignificant. At one point they have a spot elevation of about 2,800 feet, but they rarely exceed 1,500 feet elsewhere.