View source for Samudra Manthana (समुद्रमन्थनम्)

The legend of churning Milk Ocean is narrated to Rama and Lakshmana when they reach and see a city named Vishaala. Vishvamitra narrates how haalahala, the lethal poison as well as Amrita, the ambrosial elixir emerged from the churning of Milk Ocean, and how Shiva contained the poison and how Vishnu helped the churning in His incarnation as Tortoise.

Bala Kanda - Sarga 45

Once in Krita era, oh, Rama, the sons of Lady Diti were extremely energetic, whereas the sons of her younger sister Lady Aditi were vigorous and highly righteous. Oh, tigerly-man, Rama, then those great-souls speculated as to 'how we can thrive without ageing, illness, and likewise without death.' A thought occurred to those masterminds who were thinking on that matter clueing them up, 'we indeed can get elixir of life by churning the Milky Ocean. Deciding upon to churn the Milky Ocean then made Vasuki, Thousand-headed King of Snakes, as the churning rope and Mt. Mandara as stirrer, and those brothers whose energy is unlimited have started churning the Milky Ocean thoroughly. After a thousand years, the Thousand-headed serpent Vaasuki, which is being used as churning rope, is incapacitated to bear the friction of churning and fanged the cliffs of Mt. Mandara. Thereby a great lot of venom is disgorged from the heads of that serpent Vaasuki, which venom on melting the rocks of Mt. Mandara became the holocaustic poison called haalahala. A lethal poison similar to inferno known as haalahala has started to fulminate therefrom, by which whole universe of gods, non-gods and humans is burnt down. The gods seeking shelter then approached Rudra, the Cardinal God, Endower of Solace, and who husbands all the created animals inclusive of human-animals, namely Shiva, and they prayed to him saying 'save, save us.' And on observing the scare of gods and also on paying heed to the words of the Wielder of Bow called shaara~Nga, namely Vishnu, god Shiva gulped that lethal poison, haalahala , as if it is ambrosia. The god of gods Hara then left for his abode Kailash parting with other gods, and oh, Rama, the joy of Raghu's dynasty, the gods and non-gods have continued to churn the Milky Ocean. Now the loftiest mountain that is being used as stirrer, namely Mt. Mandara, has caved into the netherworld, thereby the gods along with gandharva-s have prayed to Madhusuudana, the other name of Vishnu. You alone are the recourse to all beings, especially to the dwellers in heavens, oh ambidextrous god Vishnu, you safeguard us, it will be apt of you elevate the mountain.' So prayed gods to Vishnu. Hearing the prayer of gods Vishnu, Hrisheekesha, the controller of senses, adopting the form a tortoise and positioning the stirring Mt. Mandara on that tortoiseshell, he lay recumbent at the bottom of Milky Ocean as a base to the stirrer. Keshava then reached out his hand and grasped the mountaintop as he is the Cosmic-Soul, and thus staying amid gods that Supreme Person Vishnu participated in churning the ocean by holding the stirring mountain upright. After a thousand years of churning, then a male personality, an epitome of Life Sciences, namely '''aayur veda''' , a highly righteous soul, named Dhanvantari, has firstly surfaced with his arm-rest-stick and with his handy water-vessel, and then the remarkably elegant Apsara-s, angelic damsels, have emerged next to him from the Milky Ocean. Oh, descendent of Raghu, then the heaven-sent damsel Vaaruni came up from Milky Ocean searching for her espousal, who is the daughter of Varuna, the Rain-god, and who incidentally is the presiding deity of hard liquors and also called as sura. Then a best horse called Ucchaishravaa has emerged, oh, Rama, the best among men, and then a gem of a jewel, called Kaustubha, and like that amrita, the Supreme ambrosial elixir of gods, have also emerged. Owing to the dispute regarding the possession of that ambrosia, oh, Rama, then there chanced a rampant ethnic havoc, and then the sons of Aditi have havocked the sons of Diti. All the asura-s and demons have come to one side against sura-s, and there occurred a very gruesome war which was perplexing to all the triad of universe viz., ethereal, real and surreal spheres. When everything is wading into annihilation then that omnicompetent Vishnu swiftly impounded Amrita, the Divine Elixir, by assuming his illusory power of Mohini. Whoever confronted that Eternal and Supreme Person, namely Vishnu, in that war, then Vishnu whose blaze is threefold as manifest in the sun, fire and lightning, has pulverised him. On eliminating the demonic sons of Diti and on acquiring kingdom of heaven, that eliminator of enemy cities, namely Indra, happily ruled the worlds that are inclusive of sages and caarana-s." Thus Vishvamitra continued his narration about Vishaala city and its emergence.

In Krta yuga, the sons of Diti (Asuras) and Aditi (Suras) speculated on how to thrive without ageing, illness, and likewise without death. They thought of getting amrta by churning the Milky Ocean and began by making Vasuki, the thousand-headed King of Snakes, as the churning rope and Mount Mandara as the stirrer. After a thousand years, Vasuki, unable to bear the friction of churning, let out the poisonous venom of halahala. The deities seeking shelter then approached Rudra who gulped that lethal poison as if it was amrta. The Suras and Asuras then continued to churn the Ocean. Now, the loftiest mountain being used as stirrer, caved into the netherworld. The deities along with gandharvas then prayed to Vishnu who adopted the form of a tortoise and positioning Mandara on the tortoiseshell, lay recumbent at the bottom of the Ocean as a base to the stirrer. After a thousand years of churning, Dhanvantari first surfaced followed by the Apsaras, Varuni, Ucchaishrava, Kaustubha, etc. and the amrta. Owing to the dispute regarding the possession of the amrta, a very gruesome war occured. Vishnu then swiftly impounded the amrta by assuming the form of Mohini.

This story is narrated to Rama and Lakshmana by Vishvamitra and appears in the Balakanda (Sarga 45) of the Ramayana.