The eleventh incarnation of the Lord took the form of a tortoise whose shell served as a pivot for the Mandarācala Hill, which was being used as a churning rod by the theists and atheists of the universe.
Srimad Bhagavata Purana
श्रीमद्भागवतमहापुराणम्
मूल श्लोकः
सुरासुराणामुदधिं मथ्नतां मन्दराचलम् । दध्रे कमठरूपेण पृष्ठ एकादशे विभुः
surāsurāṇāmudadhiṃ mathnatāṃ mandarācalam | dadhre kamaṭharūpeṇa pṛṣṭha ekādaśe vibhuḥ
SB 1.3.16