surasuranam udadhim
mathnatam mandaracalam
dadhre kamatha-rupena
prstha ekadase vibhuh
SB 1.3.16

Translation
The eleventh incarnation of the Lord took the form of a tortoise whose shell served as a pivot for the Mandaracala Hill, which was being used as a churning rod by the theists and atheists of the universe.

Purport by Srila A.C. Bhaktivedanta Swami Prabhupada
Once both the atheists and the theists were engaged in producing nectar from the sea so that all of them could become deathless by drinking it. At that time the Mandaracala Hill was used as the churning rod, and the shell of Lord Tortoise, the incarnation of Godhead, became the resting place (pivot) of the hill in the seawater.