A mighty angel: see 5.2; 10.1.

A stone like a great millstone: “a stone that was as big as a large millstone.” This refers to the large upper stone, turned by an animal to grind grain. Where millstone is unknown the translation can be “a very large stone.”

The sea: see 10.2.

So shall Babylon the great city be thrown down with violence: the Greek word translated violence means “with an impulsive rush,” like that of an unruly crowd or a herd of pigs, and appears only here in the New Testament. In languages where “to throw down” does not apply naturally to the destruction of a city, the translation may have to say “With violence like this the mighty city Babylon will be destroyed” or “This is how God will use great violence to destroy the great city Babylon.”

Shall be found no more: see the similar expression in verse 14: “will never reappear,” “will be gone forever,” or “people will never see this city again.”

In verses 22-23 the angel addresses the city, using the second person singular. It may be well to do the same here in verse 21, as follows: “Babylon, you mighty city, this is how you will be violently destroyed! Mighty city, you will disappear forever!” or in languages that do not use the passive, “Babylon, you mighty city, this is how God will destroy you with great force! Mighty city….”

Quoted with permission from Bratcher, Robert G. and Hatton, Howard A. A Handbook on The Revelation to John. (UBS Handbook Series). New York: UBS, 1993. For this and other handbooks for translators see here .