21Then a mighty angel took up a stone like a great millstone and threw it into the sea, saying, “So will Babylon the great city be thrown down with violence, and will be found no more;
The point is to make a comparison. God will do to (fallen) Babylon precisely what John saw the angel do to the millstone: heave it away so that it is irretrievably gone.
21 Then a mighty angel took up a stone like a great millstone and threw it into the sea, saying, “So will Babylon the great city be thrown down with violence, and will be found no more;