The question of the world’s biggest lake is complex because the answer depends on the criteria used to define “biggest.” The distinction often comes down to whether the measurement is based on surface area or total water volume. Classification is also complicated by whether the water body is considered saline or freshwater. These varying definitions lead to different, yet equally valid, contenders for the title of the world’s largest lake.
The World’s Largest Lake by Area
By the most common measure—surface area—the largest inland body of water is the Caspian Sea. It spans approximately 371,000 square kilometers, nearly five times the size of the next largest lake. The Caspian Sea is classified as a lake because it is entirely landlocked, lacking a natural outflow connection to the global ocean system. This makes it an endorheic basin, where water loss occurs primarily through evaporation.
The historical term “sea” stems from its vast size and brackish, or salty, water. Its salinity is roughly 1.2%, about one-third the salt concentration of the average ocean. This salinity fuels the debate over whether it should be called a sea or a lake. Geographically, it holds the undisputed title of the largest lake on Earth by surface area.
Geographic and Geologic Context
The Caspian Sea is situated in the depression between Europe and Asia, bordered by five littoral states: Russia, Kazakhstan, Turkmenistan, Iran, and Azerbaijan. It stretches approximately 1,200 kilometers from north to south, with an average width of 320 kilometers. Its surface sits about 27 meters below sea level, reflecting its unique geological history.
The sea originated millions of years ago from the ancient Paratethys Sea, a remnant of the Tethys Ocean. Continental collision isolated a portion of the Paratethys, forming the landlocked basin. The sea is divided into three regions: the shallow Northern Caspian, the Middle Caspian, and the Southern Caspian. The Northern section, fed primarily by the Volga River, is extremely shallow, averaging only 5 to 6 meters deep. However, the Southern Caspian reaches a maximum depth of 1,025 meters, giving it a water volume of approximately 78,200 cubic kilometers.
Unique Salinity and Ecosystem
The water chemistry of the Caspian Sea is not uniform, featuring a salinity gradient due to the inflow from the Volga River in the north. The northernmost part is the freshest, while the southern and eastern regions are saltier due to lower river input and higher evaporation. This brackish environment supports a unique ecosystem adapted to fluctuating conditions.
The Caspian Sea is home to numerous endemic species, including the small Caspian seal, one of the few seals that lives entirely inland. It is also renowned for its sturgeon populations, including the beluga, stellate, and Russian sturgeon, which are prized for their roe (caviar). These anadromous fish spend their adult lives in the sea but migrate up inflowing rivers to spawn, making them vulnerable to habitat destruction and overfishing. These life forms are remnants of the fauna that inhabited the ancient Paratethys Sea.
Defining “Biggest”: Freshwater Contenders
Considering the ambiguity of “biggest lake” requires examining the largest freshwater bodies, depending on whether area or volume is prioritized. If the definition is restricted to freshwater lakes by surface area, the title belongs to Lake Superior in North America. Bordering the United States and Canada, Lake Superior covers about 82,100 square kilometers, making it the largest freshwater lake by that metric.
If the measure of “biggest” is shifted to volume, which accounts for depth, the answer is Lake Baikal in Siberia, Russia. Although its surface area is smaller than Lake Superior’s, Baikal is the deepest lake in the world, reaching a maximum depth of 1,642 meters. This immense depth allows it to hold more than 20% of the world’s total unfrozen fresh surface water, giving it the largest volume of any freshwater lake.