Kurma , or Turtle, is the second incarnation of Hindu God Vishnu. Hindu devotees also refer to this avatar as Adi-Kurma and the Tortoise incarnation. This Avatar was taken by Vishnu to help the Devas acquire the Amruta, or elixir of life, during the Samudra Manthan or churning of the ocean.
The information about the Kurma Avatar is mainly found in the Bhagavad Purana (Canto 8 Chapter 7), Agni Purana and Ramayana (Balakanda Chapter).
The Asuras (demons) and Devas (demigods) approached the Milky ocean to perform the churning. Mandara Mountain was chosen as the churn drill. The huge snake Vasuki volunteered to be the Churning rope.
The Devas took hold of the tail of Vasuki and the Asuras took hold of the mouth the snake. But as Mandara Mountain had no fixation (below or above) it started to sink down.
Sri Hari Vishnu then took the form of a turtle or Kurma and lifted the mountain on his back and kept the mountain in the right position and made it possible for the churning of the ocean or Samudra Manthan to take place.