John Gill Commentary Revelation 16:20

"And every island fled away, and the mountains were not found." — Revelation 16:20 (ASV)

And every island fled away
This may be understood either of the dissolution of the world, the present heavens and earth, when there will be no more sea, (Revelation 21:1) and so consequently no islands:

and the mountains were not found ;
for the earth and the heaven will be fled away at the appearance of Christ the Judge, and there will be no place found for them, (Revelation 20:11) and new heavens, and a new earth, without sea or mountains, will succede: or rather this may signify the utter extirpation of all the antichristian powers in every shape, whether on islands or on the continent; for this day of the Lord will be upon every high mountain and hill, to bring them low, and the Lord alone shall be exalted, (Isaiah 2:14Isaiah 2:15Isaiah 2:17) and may particularly respect the dissolution of monasteries, and the plundering them of their riches and revenues.