In Tablet XI of the Epic of Gilgamesh, Uta-napishti relays the story of the Deluge:
Said Uta-napishti to him, to Gilgamesh:
'Let me disclose, Gilgamesh, a matter most secret,
to you will tell a mystery of gods.
'The town of Shuruppak, a city well known to you,
which stands on the banks of the river Euphrates:
this city was old - the gods once were in it -
when the great gods decided to send down the Deluge.
Source: The Epic of Gilgamesh, translated by Andrew George, Penguin Books
Why? What cause had the gods to destroy mankind?