“This legend or myth or dim memory of some ancient history has always troubled me. In sleep I had the dreadful dream of the ineluctable Wave, either coming out of the quiet sea, or coming in towering over the green inlands. It still occurs occasionally, though now exorcized by writing about it. It always ends by surrender, and I awake gasping out of deep water. I used to draw it or write bad poems about it.” ― J.R.R. Tolkien, Letter 257

Source: Atlantis – Tolkien Gateway