Water, which covers around 70 percent of the Earth's surface, comes in the forms of freshwater and saltwater. It can be found all over the world, in virtually every region no matter how dry it may appear. Knowing about some of the planet's water sources gives you some insight about this "blue planet," and how water is integral to the survival of living things.

Oceans contain more than 90 percent of the Earth's natural water reserves. Made up of saltwater, oceans are as old as the planet itself. Bodies of water such as the Pacific, Atlantic and Indian oceans are home to the most diverse array of life on the planet, and much of the planet's oceans is unexplored due to depth, darkness and temperatures.

Freshwater lakes are inland and found throughout every region of the planet. Some lakes, however, have some saltwater content, or brackish water. Like oceans, lakes are home to various forms of life, from fish and crustaceans to algae and microscopic organisms. Lakes vary in size from small and quaint to massive, such as the Great Lakes in the United States.

Rivers and streams are natural sources of moving freshwater that can be found in every country in the world. Rivers and streams can be so tiny that you can literally step over them. Or, like the Mississippi River, they can be so vast that most people couldn't swim across them. Rivers empty into bodies of water like lakes, basins, reservoirs and even the ocean. Living organisms also thrive in rivers and streams, but these natural water sources are noted for their ability to erode rocks and naturally smooth and polish them.

Water pollution is a serious issue. Human beings pollute the world's oceans and other bodies of water by emptying their wastes into lakes, rivers and seas. Some governments set strict guidelines on the amounts of wastes dumped offshore, and some restrict offshore dumping of waste altogether, but it still happens. With ever source of water on Earth being a habitat for life forms of all shapes and sizes, it's important to know the effect that pollution has on water.