And the second angel sounded his trumpet; and something like a great mountain burning with fire was cast into the sea; and a third of the sea became blood, and a third of the creatures which were in the sea which had life, died, and a third of the ships destroyed. Revelation 8: 8, 9

Looking to Jeremiah 51: 25, 42 that says Babylon is a ‘destroying mountain’ that may be made a ‘burnt mountain,’  and is ‘covered by water;” the understanding is that he’s speaking figuratively.  And so is John, when  in Rev 19: 20, 21, he speaks that “the beast was taken, and . . . cast alive into a lake of fire burning with brimstone.” The “beast” is represented in Revelation 17 as a corrupt woman whose name as “BABYLON THE GREAT.”

So, during this post-probation, pre-millennial time that the trumpet is blowing, we can know that the “great mountain burning with fire” is spiritual Babylon, a confederation of fallen churches headed by Rev. 13: 1’s sea beast, a literal nation (Vatican city). It is this sea beast that will so burn that it will contaminate the sea.