Churning of the ocean (samudramathana) is a myth, which tells how the great flood covered the earth, as a result of which many precious were lost, especially amrta, the nectar of immortality, or soma. In order to recover them Vishnu, in his incarnation (avatar) as the tortoise Kurma dived to the ocean floor so that Mount Mandara could be set up on his back. The gods and demons then coiled the serpent Vasuki around the mountain, and, by pulling on each end, churned up the ocean until the missing objects were recovered. However, a poison, Halahala, was also churned up, which Shiva drank, in order to save humanity, with a consequence that his throat is dyed blue. A version of the myth was already known in Vedic times (Rg Veda 10. 136) where it is Rudra who is the savior. Later developments took the churning to be the origin of all created things; or to be the religious process of passing through many processes and states to obtain the immortal goal of moksa, the release from the life and rebirth cycle. A.G.H.
Bowker, John, The Oxford Dictionary of World Religions, New York, Oxford University Press, 1997, p. 224