Considering how dominant water is in shaping the face of our planet, it appeared on this planet relatively slowly. Half of the world's water was generated during Earth's formative years by volcanoes spewing out steam. The other half, however, some claim was brought to the planet by giant asteroids made up of rock and ice.

Originally released by BBC Worldwide Learning, 2010. Streaming video file encoded with permission for digital streaming by Infobase on April 18, 2015.

Water World (3:00); .