The first incarnation of Lord Vishnu. There are two mythological stories connected to this avatar.According to one story, it is said that in Satya Yuga (the 1st Yuga as per Hinduism) there was a danger of the end of the world and it was predicted that all the three worlds would be submerged in ocean. So Lord Vishnu incarnated as a gigantic fish (matsya) to protect the three lokas from getting submerged and preserve the life on Earth. According to other belief, it is said that in Satya Yuga, the Vedas (the religious scriptures of the Hindus) was taken away by one Asura (demon) who hid himself and Vedas under the ocean floor. Lord Vishnu - the Hindu god of Preserver, had taken the form of a gigantic fish and dived into the depths of the ocean to get back Vedas from the asura by killing him and thus preserved Vedas.