What does Revelation 18:21 mean?

In the New Testament era, a typical millstone, usually made of basalt, was about a foot and half (45 cm) in diameter and about four inches (10 cm) thick. Two millstones, one above the other, were used to grind grain. Some were even larger, requiring animals and mechanisms to operate. Even a "small" millstone can weigh more than 100 pounds (45 kg). In John's continuing vision (Revelation 17:1–3), a mighty angel, perhaps Michael the archangel, picked up a millstone described as "great." This uses the Greek root word megas, implying something very large. A "very large" millstone could easily weigh thousands of pounds / kilograms.

After picking up the large millstone, the mighty angel threw it into the sea. He cried out loudly that Babylon would be thrown down violently and vanish from the face of the earth. Earlier, the demise of Babylon was said to have happened "in a single hour" (Revelation 18:10). This description supports the full meaning of that phrase: Babylon's ruin is sudden, dramatic, and unavoidable. Like dropping a boulder into the ocean, it happens with violence and is over as quickly as it starts.

This is precisely what happened to Sodom and Gomorrah, when God destroyed those wicked cities with sulfur and fire (Genesis 19:24–25). This rapid, total ruin happens at the end of the tribulation, when God overthrows Babylon. The once proud, powerful capital of the beast's empire (Revelation 13:1–10) becomes an ash heap and disappears from the face of the earth. Babylon had opposed God and His people. It had practiced all kinds of evil. Its ruler, the beast, assumed the role of God and demanded worship (Revelation 13:11–15), but the true God put an end to all that with one powerful stroke.