A strong angel takes up a millstone and tosses it into the sea. Just so, he says, Babylon will be thrown down violently and will never be found (Revelation 18:21).
How is a millstone like a city?
For starters, a millstone is a piece of earth, a rock that is shaped for a particular purpose. It’s a cultural artifact, and one that is useful for human production. A millstone turns nature into culture, wheat into flour, which makes it possible to turn flour into a feast of bread. When the city-millstone disappears into the sea, the secondary creation of the civic environment dissolved back into the formless-void of the original creation. A millstone thrown into the sea is like a cultural achievement collapsing into chaos.
More specifically: How is a millstone like a feminine city?
Throughout the Bible millstones are linked with women. Grinding is women’s work. There are slave girls at the millstone in Egypt (Exodus 11:5), women are taken from the millstone when the invaders snatch away the inhabitants of the land (Matthew 24:41; Luke 17:35). It was a woman who dropped the millstone that crushed the head of Abimelech (Judges 9).
From the feminine associations of millstones, we find that there are sexual associations. “May my wife grind for another,” says Job (31:10), and Isaiah 47:2 prophesies that the unfaithful city will be exposed as she is taken into slavery (veil removed, leg uncovered), and in the same context speaks of the woman grinding meal at a millstone. Babylon is a harlot who has been grinding for many men.
Jesus speaks of millstones in a different context: Anyone who offends one of the little ones should be tossed into the sea with a millstone strapped to his neck (Matthew 18:6). Babylon has not only offended, but slaughtered, the innocent lambs of Jesus’s flock, and so she becomes the millstone tossed into the sea. And, in the larger scope of Revelation, she is the mountain-city that is uprooted by the prayers of the saints—including the continuing prayers of the very martyrs she slaughters—and cast into the sea (Matthew 21:21).