There are two ways to define biggest
1. Size or Area
2. Volume
By Size
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Biggest is Caspian Sea, Asia-Europe (371,000 sq km) . But it is not a fresh water lake.
Lake Superior - 31,700 sq. mi, Is the Largest fresh water lake in the world
LAKES (By Size)
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Caspian Sea, Asia-Europe (371,000 sq km)
Superior, North America (82,100 sq km)
Victoria, Africa (69,500 sq km)
Huron, North America (59,600 sq km)
Michigan, North America (57,800 sq km)
Tanganyika, Africa (32,900 sq km)
Baikal, Asia (31,500 sq km)
By Volume
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The volume is defined by amount of water it holds.
Lake Baikal is the largest (by volume).A World Heritage Site, it lies in Southern Siberia in Russia, between Irkutsk Oblast to the northwest and Buryatia to the southeast, near the city of Irkutsk. The name derives from Tatar "Bai-Kul" - "rich lake". It is also known as the Blue Eye of Siberia.