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At the commencement of a new eon, when the Earth is covered by water in every direction, Vishnu takes the form of a huge boar (varaha). The boar dives down to the bottom of the ocean and takes the Earth to the surface again so that life can reemerge.
Further reading: Nanditha Krishna, The Book of Vishnu (New Delhi: Viking, 2001); A. K. Ramanujan, Hymns for the Drowning (Princeton, N.J.: Princeton University Press, 1981); Pandrimalai Swamigal, The Ten Incarnations: Das-vatara (Bombay: Bharatiya Vidya Bhavan, 1982).
Encyclopedia of Hinduism. A. Jones and James D. Ryan. 2007.