how much water on the earth
Water covers about 75% of the Earth's surface. The origin of water on Earth, or the reason that there is clearly more liquid water on the Earth than on the other rocky planets of the Solar System, is not completely understood.
How much water is that? It's roughly 326 million cubic miles (1.332 billion cubic kilometers), according to a recent study from the U.S. Geological Survey. Some 72 percent of Earth is covered in water, but 97 percent of that is salty ocean water and not suitable for drinking. There's not a lot of water on Earth at all, said David Gallo, an oceanographer at the Woods Hole Oceanographic Institution (WHOI) in Massachusetts. Oceans create a water layer spanning 15,000 miles (24,000 kilometers) across the planet at an average depth of more than 2 miles (3.2 km). If you poured all of the world's water on the United States and could contain it, you'd create a lake 90 miles (145 km) deep.