A lake is a landlocked mass of water. That is, it's contained by land from all sides. The word 'Largest' is vague. By 'Largest Lake', one could mean the largest by water volume or by surface area. Also, it could either contain freshwater or saline water.
The Caspian Sea
Largest Lake in the World
By all criteria, which includes the one with largest volume of water and largest surface area, the largest natural lake in the world is the Caspian sea. Yes, the biggest lake is a sea.
Its salinity is about 1.2 %, which is about one-third of the salinity of most seas in the world. The Caspian Sea has a maximum depth of 1,025 meters (3,363 feet). It is what the geologists call an endoheric basin, that is, there are no outflows from it.
It is surrounded by Iran in the North, Russia in the South, Kazakhstan and Turkmenistan in the West, and Azerbaijan in the East. The Caspian Sea, along with the Black Sea, is the remnant of an ancient ocean called the 'Paratethys Sea'.
Research in geology has revealed, that the Caspian sea became landlocked, about 5.5. million years ago because of a tectonic uplift (a lift in the crust of Earth) and subsequent fall in sea level.
According to a Greek historian Strabo, its name 'Caspian' is supposed to have been derived from the Sanskrit word 'Kashyapa', which was the name of an ancient Indian Sage. The Caspian Sea was also known by the name of Qazvin on ancient maps and is known as Darya-e-Mazandaran or Darya-e-Khazar in Persian.
Lake Baikal is also the deepest and the oldest freshwater lake (age: 25-30 million years) in the world, with a maximum depth of 1,285 meters. It is also one of the most transparent lakes in the world. It holds about 20% of the total freshwater on Earth. It is situated near the city of Irkutsk in Siberia and is known as the 'Pearl of Siberia'.
Lake Baikal is home to about 1550 species and varieties of animals, as well as 852 species of algae. The lake is fed by 300 rivers and drained through a single outlet, which is the Angara river.
Lake Michigan - Huron
Both, Lake Michigan and Lake Huron are part of the North American Great Lakes. They were formed from the melting of glaciers, from the last ice age.