Amrita is not found, gathered or grown. It has to be manufactured, and the manufacture requires the gods to work with their enemies, which is what makes the story worth telling. The devas, weakened and facing defeat, are advised that the ocean of milk holds the drink of deathlessness and that they cannot raise it alone. They propose an alliance to the asuras on the promise of an equal share.
The apparatus is cosmic. The mountain Mandara becomes the churning stick and the serpent Vasuki the rope wound round it, with the asuras taking the head and the devas the tail. Vishnu takes the form of a tortoise and goes under the mountain so that it will not sink. The churning brings up treasure after treasure before it produces what it was for: the moon, the wish granting cow, the celestial physician's art, the goddess Sri. It also brings up the poison halahala, which threatens everything alive until Shiva drinks it and holds it in his throat, which is why he is called the blue throated one.
Last comes Dhanvantari, rising out of the water with the vessel of amrita in his hands. The alliance ends there. The asuras seize the vessel, and Vishnu takes the form of Mohini, a woman of such beauty that they agree to let her serve it out fairly, and she gives it to the devas alone. One asura slips into the line in disguise and drinks; the sun and moon see him and tell her, and she takes off his head before the amrita passes his throat. The head lives, because the drink had already reached it. That is Rahu, who swallows the sun and the moon in revenge and lets them go again, which is what an eclipse is.