The natural reserve encompasses the larger part of the Pirin mountain in the south-west of Bulgaria. The protected territory covers about 26, 000 ha. The Thracians called the mountain Orbellus (“Snow Mountain”), and the Slavs associated it with their god of storm and thunder Perun, hence the present-day name. According its geological structure, Pyrin is a horst with a granite core, covered mainly with old metamorphic rock.
The mountain was formed in the middle of the Tertiary. At the beginning of the Quaternary the rising of the mountain coincides with its icing. Many circus lakes (176; Bandersihki, Popovi etc), glacier valleys, moraines etc were formed then. Above 2, 300 m there is a constant snow-cap. The mountain has Alpine relief and mountain climate, with Mediterranean influence along the valleys of the rivers Struma and Mesta. Vihren (2,914 m) is the highest among the 61 peaks above 2, 500 m. There are many rivers and streams, the sources of the rivers Banderishka, Demianitsa, Sandanska Bistritsa etc, and more than 70 thermal springs.
Forests cover more than half the area of the mountain and comprise all coniferous species of Bulgaria, as well as many deciduous. The white and black furs are an example, the Baykusheva fur being the oldest known tree. It is around 1, 300 years old, 16 m high and has a circumference of 5.72 m. All trees more than 280 years of age have been declared natural phenomena. Plants characteristic of the height above 2, 000 m. are: pine-scrub, Siberian juniper, heath-berry, the spotted gentian, etc. At greater heights, Pyrin poppy, edelweiss and willow can be found.
Rare animal species inhabiting Pyrin include wild goats, wild cats, deer, boar, wolves, bears, foxes, martens, squirrels, and rare birds, among which the almost-extinct wood-grouse. There is a great variety of butterflies. Endangered species include the Greek turtle, the speckled polecat, the brown bear, the wild cat, the wild goat, the rock eagle, the wood-grouse, the owl, the speckled woodpecker, etc.
113 species of the park’s flora and fauna are included in the Red Book. In order to provide protection for the unique variety of landscape, plant and animal species.
The Pirin National Park was included in the World Cultural and Natural Heritage List at the World Cultural and Natural Heritage Committee session of 1983 in Florence, Italy.