A lake is water that has land all around it. Some lakes are so big that we can’t see the other side. Lake Superior in North America is the largest freshwater lake in the world. It spreads over more than 82,100 square kilometres.

Some areas of water called seas are actually lakes because land surrounds them. The Caspian Sea, for example, is the world’s largest saltwater lake. This lake, which lies between Europe and Asia, stretches for 372,000 square kilometres.

Most lakes are just holes in the ground that are filled with water. Glaciers dug many of these holes. Long ago, these huge rivers of ice flowed out of the north and covered many parts of the world. As the gigantic glaciers slid slowly along, they cut out great pits and made valleys wider and deeper. Then, when the glaciers began to melt, the water filled up many of the holes, forming lakes.

Some lakes form when part of the earth caves in, leaving a hole. This happens mostly in places where the ground is limestone. Year after year, rain dissolves away the soft limestone, forming caves and tunnels.

Finally, the tops of these tunnels cave in, leaving what is called a sinkhole. Rain or water from underground springs and streams fills the sinkhole, and it becomes a lake.

Part of a river can also become a lake. Sometimes a river deposits so much mud and sand that the water backs up and forms a natural lake. People may make a lake by building a dam. A dam causes the flowing water to spread out over the river’s banks and form a lake.

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