§ Of the Beginning of Days The Silmarillion

In the confusion and the darkness Melkor escaped, though fear fell upon him; for above the roaring of the seas he heard the voice of Manwë as a mighty wind, and the earth trembled beneath the feet of Tulkas. But he came to Utumno ere Tulkas could overtake him; and there he lay hid. And the Valar could not at that time overcome him, for the greater part of their strength was needed to restrain the tumults of the Earth, and to save from ruin all that could be saved of their labour; and afterwards they feared to rend the Earth again, until they knew where the Children of Ilúvatar were dwelling, who were yet to come in a time that was hidden from the Valar.