This chapter offers insights into a long-term research project that seeks to distinguish between myths and their various manifestations in literary sources, and thus approaches Mesopotamian mythology as a body of sacred, oral stories that lie in the background both of texts and of other forms of cultural expression, in this instance the work of the Babylonian priest and historian Berossus, in particular Book I of his Babyloniaca, in which he summarised Babylonian cosmogonic beliefs for a Greek readership. While the links between this part of the Babyloniaca and the Akkadian poem Enūma eliš are well-known, Berossus combined knowledge of that text with a Mesopotamian myth of origins on the primeval pair ‘Father Sky and Mother Earth’ that was never fixed in writing. Taken together with sporadic evidence from Sumero-Akkadian sources collected by the author, the Babyloniaca emerge as an important source on this influential but elusive myth, which was overshadowed without being fully supplanted by the Marduk-centred theology of Enūma eliš.