Lord Vishnu presiding over the churning of the ocean.
Halfway through the churning, Mount Madura which was
used as a churning stic, started to sink. Lord Vishnu
assumed the form of his turtoise avatar Koorma, dived
to the bottom and let them use his back as a pivot upon
which the churning stick could turn.

You can also see both the demons and the gods on either
sides, holding on to the serpent Vasuki.