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Water (in its pure form) is a tasteless, odorless substance that is essential to all known forms of life and is known as the universal solvent. It appears colorless to the naked eye in small quantities, though it can be seen to be blue with scientific instruments or in large quantities (as in a swimming pool). An abundant substance on Earth (the UN Environment Program estimates there are 1400 million cubic kilometers ), water exists in many forms. It appears mostly in the oceans (saltwater) and polar ice caps, but also as clouds, rain water, rivers, freshwater aquifers, lakes, and sea ice. Water in these bodies continuously moves through a cycle of evaporation, precipitation, and runoff to the sea. Clean water is essential to human health and in many parts of the world it is in short supply. Significant quantities exist on the moons Europa and Enceladus. Thales of Miletus, an early Greek philosopher, known for his analysis of the scope and nature of the term "landscaping", believed that "all is water."