Nemrut Crater Lake is a freshwater crater lake located within the caldera of Mount Nemrut in Türkiye. It is situated within the borders of Tatvan district of Bitlis province, west of Lake Van. The Nemrut Stratovolcano where the lake is located, is situated at approximately 38°37′ 10′′ N latitude and 42°14′ 28′′ E longitude.

^{Nemrut Crater Lake (AA)}

Nemrut Volcano and Caldera

Mount Nemrut is a stratovolcano that was active in historical times, with its last recorded lava flow in 1441 AD. Fifteenth-century records indicate that a major eruption occurred on the mountain in 1441, accompanied by loud noises, the mountain splitting open, and the emission of fire, dense smoke, and a strong odor into the surroundings. Written sources also indicate volcanic activity in 1411. The presence of two distinct pyroclastic layers on the crater rim geologically supports that these two eruptions occurred approximately 30 years apart. Although considered a dormant volcano, Nemrut is not believed to be completely extinct due to the presence of a hot lake (Ilıgöl) within the caldera and ongoing hot gas emissions.

The caldera of Mount Nemrut measures approximately 7.5 km along its east-west axis and 6 km along its north-south axis. It is a large and elliptical depression. Its formation is attributed to the collapse of the volcano's summit following eruptive periods. The current caldera shape is thought to have formed as a result of a major eruption approximately 100,000 years ago, which significantly reduced the mountain's height. The caldera floor is at an elevation of 2247 m above sea level and is surrounded by steep slopes 500-600 m high. The eastern part of the caldera is partially filled with volcanic products from post-collapse eruptions. Within the caldera, there are five lakes, numerous lava extrusion centers, lava cones, spatter cones, hot springs, and six caves. Volcanic materials in the region include scoria, pumice, obsidian, basalt, andesite, and trachyte, as well as pyroclastic rocks such as ash, tuff, and ignimbrite. Peaks on the caldera rim include Sivritepe (2935 m), East Nemrut Peak (2625 m), Tursuktepe (2828 m), and Nemrut Mountain Peak (2801 m).

Nemrut Crater Lake (Main Lake)

Nemrut Crater Lake is Türkiye's largest crater lake and has been identified as the world's second-largest volcanic lake. It occupies the crescent-shaped western part of the caldera.

• Elevation: The lake surface is at an elevation of 2247 m above sea level.
• Dimensions: Its surface area has been reported as 10 km2 or 12.52 km2; some sources indicate it can extend up to 15 km2. The lake is approximately 6 km long and 2 km wide; other dimensions given are 4.8 km (north-south) and 2.7 km (east-west).
• Depth and Volume: The maximum depth of the lake has been determined to be 176 m. The total water volume is approximately 1.25 km3.
• Water Characteristics: Its water is fresh and cold. Analyses have shown the water to be clear, colorless, odorless, with the taste of normal drinking water and a pH value of 7.4 (slightly alkaline). Radioactivity levels are within normal limits. The lake maintains its level primarily through rainfall and some surrounding springs.
• Ecology: The lake is rich in phytoplankton. Mirror carp introduced to the lake in 1986 have formed a significant population and enabled fishing activities. The endemic fish species Barbus ercisianus, inhabiting the lake, has been found to host the parasite Ligula intestinalis, first reported in this region by Önalan and colleagues in 2022.

^{Main Lake (AA)}

Ilıgöl (Hot Lake)

Ilıgöl, located near the northern tip of Nemrut Lake, is a roughly circular lake with a diameter of approximately 500 m and a maximum depth of 10 m. Its surface area is approximately 1.2 km2. Although once thought to be connected to the main Nemrut Lake, it is now separated by small lava piles, but seepage between them ensures water levels remain equal.

• Temperature and Composition: Due to hot water mixing from the lakebed and its surroundings, Ilıgöl's temperature can reach up to 40°C in winter and 60°C in summer. It has a high dissolved mineral content (1758.4 mg/L) and generally does not support aquatic life, except for some parasites.
• Uses and Geothermal Potential: The waters of Ilıgöl are traditionally used in the treatment of rheumatism. The hot steam emanating from cracks near the lake is also believed to be beneficial for ailments such as asthma and bronchitis. These thermal characteristics indicate geothermal energy potential, and the General Directorate of Mineral Research and Exploration (MTA) has conducted drilling operations in the area, the results of which are still confidential. The hot steam is believed to originate not directly from magmatic sources but from heated groundwater.

^{Hot Lake (AA)}

Climate and Environment

The Nemrut caldera has a microclimate described as cool and humid. The average annual temperature is 5.7°C; the coldest month is January (average −5.9°C), and the warmest month is July (average 18.5°C). The region receives a significant amount of precipitation, reaching a total of 1450 mm annually at Sivritepe. Although the surroundings are generally described as poor in vegetation, it is surprising that 4.6% of the natural vegetation within the caldera belongs to Mediterranean flora.

Lavas erupted during the formation of Mount Nemrut played a role in blocking the ancient Murat Valley, contributing to the formation of Lake Van to its east.