There was a mention of "Samudra Manthan" (the churning of the ocean) in Hindu mythology. Why was it performed? Please also mention the items which resulted from the churning, with a small explanation for each.
As goes with any Hindu myth, the culprit of the entire mayhem was Lord Indra.
Once upon a time Indra who was riding his elephant came across Durvasa Rishi, who gifted him a garland of flower. Indra, who absent mindedly instead of keeping the garland himself, put it on the elephant.
Since the garland was made of flowers, insects and bees started to get attracted by it, which provoked the Elephant who threw the garland into the floor and stomped it.
On knowing this, Durvasa was furious (because apparently, the garland was a dwelling of fortune and should have been treated with care and treated as prasadam) and went on to curse all the devas and made them dovoid of any strength, vigor and fortune.
The the Asuras knowing the ill fortune of devas captured all three lokas. Devas went to Lord Vishnu for his advice who asked them to get the Amrita (अमृत) from under the vast ocean kṣīrasāgara (क्षिर्-सगर) by Samudramanthan समुद्रमन्थन् . Devas alone weren't capable to do such a Herculean task and they needed the help of Asuras in doing so.
Asuras being naive agreed to this and they agreed to distribute the things which emerge from the sea equally among each other. Vasuki (वसुकि), King of Serpents was made the churning rope, and Mount Mandar (मन्दार) was made the spindle.
Asuras were made to hold the head of Vasuki while the Devas held on to the tail part, and hence the great churning started. During churning, Mount Mandar became unstable and was going under the sea bed. Vishnu took Kurma-Avatar (कूर्मावतारम्) (Vishnu in Tortoise avatar) and went under mount Mandar mountain to support it.
The following things came from the churning of sea,
- Sura or Varuni - Goddess and creator of wine
- Apsaras - various divine nymphs like Menaka and Rambha etc.
- Kaustubha - a rare diamond said to be the most valuable jewel in the world
- Uchhaishravas - the divine seven headed white horse
- Kalpavriksha - the wish-granting tree
- Kamadhenu - the wish fulfilling cow
- Airavata - the white elephant
- Lakshmi - the Goddess of Fortune and Wealth. Vishnu and Her were reunited after having been separated for many ages.
- Halahala - the Great Poison which threatened the very existence of World and was nutralized by Lord Shiva by drinking it.
- Amritam - The Elixir of youth and immortality.
During fight between Devas and Asuras for obtaining Amritam, a few drops fell on earth at Ujjain, Nasik, Allahabad, and Haridwar. The drops are said to have purified the land and it is here that every year devotees come to wash away their sins in the famous assembly called Kumbh Mela.