During one day of Brahma there are fourteen Manus, and at the end of each Manu there is devastation up to the earthly planets, and the vast water is fearful even to Brahma. So in the beginning of the would-be Vaivasvata Manu, such devastation would be seen by him. There would be many other incidents also, such as the killing of the famous Sankhasura. This foretelling is by the past experience of Brahmaji, who knew that in that fearful devastating scene, the Vedas would come out of his mouth, but the Lord in His fish incarnation not only would save all living entities, namely the demigods, animals, men and great sages, but would also save the Vedas.