The lake is a Ramsar site of globally significant biodiversity (Ramsar Site RDB Code 2KG001) and forms part of the Issyk-Kul Biosphere Reserve.
Issyk-Kul Lake is 182 kilometres (113 mi) long, up to 60 kilometres (37 mi) wide, and its area is 6,236 square kilometres (2,408 sq mi). It is the second largest mountain lake in the world behind Lake Titicaca in South America. It is at an altitude of 1,607 metres (5,272 ft), and reaches 668 metres (2,192 ft) in depth.
About 118 rivers and streams flow into the lake; the largest are the Djyrgalan and Tyup. It is fed by springs, including many hot springs, and snow melt. The lake has no current outlet, but some hydrologists hypothesize that, deep underground, lake water filters into the Chu River. The bottom of the lake contains the mineral monohydrocalcite: one of the few known lacustrine deposits.
The lake's southern shore is dominated by the ruggedly beautiful Teskey Ala-Too Range of the Tian Shan mountains. The Kungey Alatau of the Tian Shan runs parallel to the north shore.
The lake water's salinity is approx. 0.6%— compared to 3.5% salinity of typical seawater— and, although the lake level is still currently some 8 metres (26 ft) higher than in medieval times, its level now drops by approximately 5 cm per year due to water diversion.
Administratively, the lake and the adjacent land are within Issyk-Kul Region of Kyrgyzstan.
During the Soviet era, the lake became a popular vacation resort, with numerous sanatoria, boarding houses and vacation homes along its northern shore, many concentrated in and around the town of Cholpon-Ata. These fell on hard times after the break-up of the USSR, but now hotel complexes are being refurbished and simple private bed-and-breakfast rentals are being established for a new generation of health and leisure visitors.
The city of Karakol (formerly Przhevalsk, after the Russian explorer Przhevalsky who died there) is the administrative seat of Issyk-Kul Region of Kyrgyzstan. It is near the east tip of the lake and is a good base for excursions into the surrounding area. Its small old core contains an impressive wooden mosque, built without metal nails by the Dungan people, and a wooden Orthodox church that was used as a stable during Soviet times (see state atheism).
Issyk-Kul Lake was a stopover on the Silk Road, a land route for travelers from the Far East to Europe. Many historians believe that the lake was the point of origin for the Black Death that plagued Europe and Asia during the early and mid-14th century. The lake's status as a byway for travelers allowed the plague to spread across these continents via medieval merchants who unknowingly carried infested vermin along with them.
The lake level is some 8 metres (26 ft) higher than in medieval times. Divers have found the remains of submerged settlements in shallow areas around the lake. In December 2007, a report was released by a team of Kyrgyz historians, led by Vladimir Ploskikh, vice president of the Kyrgyz Academy of Sciences, that archaeologists have discovered the remains of a 2500-year-old advanced civilization at the bottom of the Lake. The data and artifacts obtained suggest that the ancient city was a metropolis in its time. The discovery consisted of formidable walls, some stretching for 500 metres (1,600 ft) and traces of a large city with an area of several square kilometers. Other findings included Scythian burial mounds eroded over the centuries by waves, and numerous well-preserved artifacts, including bronze battleaxes, arrowheads, self-sharpening daggers, objects discarded by smiths, casting molds, and a faceted gold bar that was a monetary unit of the time.
Articles identified as the world's oldest extant coins were also found underwater with gold wire rings used as small change and a large hexahedral goldpiece. Also found was a bronze cauldron with a level of craftsmanship that is today achieved by using an inert gas environment.
In 1916 the monastery at Issyk Kul was attacked by Kyrgyz rebels, and seven monks were killed.
The first nature reserve in Kyrgyzstan, Issyk-Kul State Reserve was established in 1948 to protect unique nature landscapes and waterfowl at Issyk Kul. In 1975, it was acknowledged as a Ramsar site. Biosphere Reserve Issyk Kul covered by UNESCO World Network of Biosphere Reserves was established in year 2000 within the administrative borders of Issyk-Kul Region.
The lake contains highly endemic fish biodiversity, and some of the species, including four endemics, are seriously endangered. In recent years catches of all species of fish have declined markedly, due to a combination of over-fishing, heavy predation by two of the introduced species, and the cessation of lake restocking with juvenile fish from hatcheries. At least four commercially targeted endemic fish species are sufficiently threatened to be included in the Red Book of the Kyrgyz Republic: Schmidt's dace (Leuciscus schmidti), Issyk-Kul dace (Leuciscus bergi), marinka (Schizothorax issyk-kuli), and sheer or naked osman (Diptychus dybovskii). Seven other endemic species are almost certainly threatened as by-catch or are indirectly impacted by fishing activity and changes to the structure and balance of the lake's fish population.
Sevan trout, a fish endemic to Lake Sevan in Armenia, was introduced into Issyk-Kul in the 1970s. While this fish is an endangered species in its "home" lake, it has a much better chance to survive in Lake Issyk-Kul where it has ravaged the indigenous species.
In pre-Islamic legend, the king of the Ossounes had donkey's ears. He would hide them, and order each of his barbers killed to hide his secret. One barber yelled the secret into a well, but he did not cover the well afterwards. The well water rose and flooded the kingdom. The kingdom is today under the waters of Issyk-Kul. This is how the lake was formed, according to the legend. Other legends say that four drowned cities lie at the bottom of the lake. Substantial archaeological finds indicating the presence of an advanced civilization in ancient times have been made in shallow waters of the lake.
Russian Navy test site
During the Soviet period, the Soviet Navy operated an extensive facility at the lake's east end, where submarine and torpedo technology was evaluated. In March 2008, Kyrgyz newspapers reported that 866 hectares (2,140 acres) around the Karabulan Peninsula on the lake would be leased for an indefinite period to the Russian Navy, which is planning to establish new naval testing facilities as part of the 2007 bilateral Agreement on Friendship, Cooperation, Mutual Help, and Protection of Secret Materials. The Russian military will pay $4.5 million annually to lease the area. India also plans to invest in the facility to test all types of torpedoes such as heavy weight torpedoes and those that have thermal navigation system. Another advantage that works for the testing center is that the torpedoes fired can also be recovered allowing scientists to make physical verification of a torpedo structure for further study. India is also planning to use the torpedo test facility to test the Autonomous Underwater Vehicle being developed by NSTL. For this India has proposed to engage local companies with know-how in torpedo technology to further co-develop the facility.
Towns and some villages around the lake, listed clockwise from the lake's western tip:Balykchy (the railhead at the western end of the lake)
Cholpon-Ata (the capital of the north shore)
Karakol (the provincial capital near the eastern end of the lake, formerly named Przhevalsk)