This work is in the public domain in the United States because it was published (or registered with the U.S. Copyright Office) before January 1, 1926.
An Angel throws a large stone or millstone into the depths of the Sea, to symbolize that Babylon is truly gone forever and will not rise again. The world's systems of power, greed, corruption and perversion will not be able to control mankind ever again. We can't comprehend what it will be like to free of such pervasive oppression, but God is making a promise that what happened on earth will not be allowed to happen again.