The second incarnation of Lord Vishnu was Kurma or Kachapa, the giant tortoise, who supported the mighty Mandara mountain during the Samudra Manthan (churning of the Ocean of Milk) to extract nectar (Amritam).
The Kurma Avatar was taken by Lord Vishnu to aid God’s victory. According to the puranic story, the gods (devatas) and asuras (demons) began churning the milky ocean to extract nectar (Amrit) that promises immortality and was hidden away at the bottom.
Both the devatas and the asuras churned the ocean using the serpent Vasuki as the rope. At the start, playing a Machiavellian trick, Indra, king of the gods asked the asuras for the head end of Vasuki. But asuras suspecting foul play, took the head end, only to be deceived as the poison from Vasuki was slowly weakening them.
They were using a mountain Mandara as a churning stick, and just as the mountain began to sink into the ocean, Lord Vishnu took the form of a Kurma to carry the weight of the mountain on its back and helped keep the land afloat.
As soon the bowl of Amrit came out the asuras grabbed it, then Lord Vishnu took the form of a beautiful maiden Mohini (apsara) and seduce the asuras to distribute the nectar. Soon after the devatas were served the maiden disappeared deceiving the asuras in order to give the immorality to the devas and that it would be dangerous for the asuras to become immortal and making them totally weak.
Lord Vishnu was the preserver that had taken the form of the tortoise and helped devatas in getting Amrit by luring the asuras by transforming Himself into a beautiful woman named Mohini.
Moral Principle behind this episode of Kurma Avatar is of immense value and worth following by all human beings. The tortoise has the unique quality of withdrawing all its limbs inside and stand like a rock, when faced with a dangerous situation.
Similarly, Lord sets an example by taking the Kurma Avatar to lift and support the submerged mountain on His Back to help the Gods and Demons to accomplish a common noble purpose of extracting the Nectar.