Being the largest and most beautiful out of Macedonia's three tectonic lakes, Lake Ohrid is about 30 kilometers (18 miles) long and around 288 meters (945 feet) deep. Its astonishingly clean and clear waters, together with the serene stillness of its mountain settings have captivated visitors since prehistoric times. While the lake is filled up by water from three rivers, most of Ohrid's water comes from another lake - Prespa which is located on the other side of Mountain Galichica. Due to the high elevation, Prespa spills its water down to Ohrid through mountain springs, the most important ones being Ostrovo and Biljana, located near the monastery of St. Naum, respectively. With its unique flora and fauna characteristic of the tertiary period (2-4 million years ago), Ohrid is one of Europe's great biological reserves. Most of the lake's plant and animal species are endemic and unique to Ohrid. The most famous among these are two types of the Ohrid trout, named letnica and belvica. Other unique Ohrid creatures include two types of eel, and the bleak whose scales are used for making the well-known Ohrid pearl. This treasured jewel is produced according to a secret method which was passed on from generation to generation. Sport fishing attracts many passionate fishermen from Macedonia, Europe and even from the world. In 1980, the Lake Ohrid was proclaimed by UNESCO as a locality of the world's cultural and natural inheritance.