Kurma means turtle is the second avatar of Vishnu. This incarnation also occurred in Satya Yuga. As per the Puranas the legend goes as below.
There was a rishi named Durvasa (believed to be an incarnation of Lord Shiva), known for his anger. The wise Durvasa gave a garland to Lord Indra who is known to be the king of Gods. Lord Indra positioned the garland around his elephant but the elephant crushed the garland. As a result, Durvasa felt insulted and cursed the gods to lose their immortality and divine powers. Gods lost their kingdom of heaven, and approached Lord Vishnu for help.
Lord Vishnu then advised them to churn the ocean of milk using Mount Mandara as the churning stick and serpent Vasuki as the rope. But as the churning started, the mountain started sinking. Lord Vishnu took the form of the tortoise or Kurma and kept the mountain afloat. As soon as the bowl of amrita was full, the nectar of immortality was out, along with fourteen treasures.
These fourteen magnificent treasures were Kalpavriksha, Kamadhenu, Chintamani, Ucchaishrava, Airavata, Panchajanya, Bow of King Saranga, Rambha, Chandra, Varuni, Dhanvantari, Goddess Sri and Halahal. As a result of the churning, the asuras (demons) got hold of the treasure Amrita and became powerful. Post this, Lord Vishnu took the form of Mohini to lure them and made the asuras weak.
Gods regained their powers of immortality and were then able to go back to their kingdom of heaven. Lord Vishnu revived their lost glory and brought judgement to the world.