A Great Millstone
And a mighty angel took up a stone like a great millstone, and cast it into the sea, saying, Thus with violence shall that great city Babylon be thrown down, and shall be found no more at all. (Rev. 18:21 KJV)
A mighty angel casts a millstone into the sea to show that Babylon will soon be destroyed. This fulfills what was foretold in Jeremiah.
63 And it shall be, when thou hast made an end of reading this book, that thou shalt bind a stone to it, and cast it into the midst of Euphrates:
64 And thou shalt say, Thus shall Babylon sink, and shall not rise from the evil that I will bring upon her: and they shall be weary. Thus far are the words of Jeremiah. (Jer. 51:63-64 KJV)