The prophet Jeremiah took a “millstone” and bound it to his book. After he had read it, he cast
it into the Euphrates River river as a sign and token of the destruction of old Babylon (Jeremiah 51:63). Likewise, the "millstone" thrown into the sea by the mighty angel (v. 21) expresses the sheer violence, force and power that will destroy Babylon leaving no possibility of ever recovering.