Jeremiah 51:25 - Matthew Poole's English Annotations on the Holy Bible

Babylon is not here called a mountain because it was situated upon any hills or mountains, for it appears from Genesis 11:2 that it was situated in a plain, and we read, Jeremiah 51:13, that it dwelt upon many waters; but because it was very high for its power and greatness, and had very high walls and towers, that it looked at a distance like a high rocky mountain, and, as some say, (being a very large city,) was full of trees. They had destroyed many people of the earth that lay near to them. God threatens to destroy them notwithstanding their towers and great fortifications, as many times they threw down malefactors from high rocks, mountains, and precipices; and to make them like \'c6tna or Vesuvius, or like some other mountains of sulphur, or other bituminous matter fired, which are always burning; or else he threateneth that their cities and towers, which appeared like a mountain, should be burnt.