Revelation 8:8 could be referring to a strike by an asteroid or comet or another large object from space (possibly one not yet identified), or it could be talking about the mushroom cloud from a nuclear explosion (a 1st-century writer could easily describe that as a mountain of fire), or it could be a direct act of God.
In any case, God is in control of whatever it is. Maybe He's going to use the forces of nature (forces that He created). Maybe He will use man's sin as man goes to war against man. Maybe He will intervene directly and miraculously. However He does it, the goal will be to punish mankind.
Beyond that, we're not really told a lot. It's interesting to speculate about what some of these things might be, but we have to recognise that we're just guessing.
There are several reasons this stuff is recorded in Revelation:
1. For us to see that God is sovereign, and is in control of this stuff and how the world will end, and that He will judge evil
2. For us to see that He will keep His promises - this was a big deal in the early church after the Romans destroyed Jerusalem and scattered the Jewish people and the Jewish nation seemed to be destroyed - had God forgotten His promises to Abraham? No, He has not.
3. For the people who will live in that time, they will be able to see prophecy being fulfilled.
We don't need to work out all the exact details for any of those. When the time comes, it will be very clear.
99942 Apophis has been cited many times as the “likely” asteroid that will be the 2nd trumpet in Revelation 8:8. Apparently this will be a narrow miss in 2029 that, if a direct impact occurs, has the ability to kill “untold millions” according to science websites.
NASA currently says the chances of a collision are so infinitesimal that we might as well forget about it. However, the possibility does indeed exist, and God is in control. The asteroid will make another close pass in 2036, then from 2060 til 2110 about 10 more close passes will occur.
Current estimates suggest that reliable defenses which protect against asteroids will be constructed in our lifetime. This may suggest the asteroid must hit no later than 2050 for the 2nd trumpet to even occur. However, after thinking some time, I realized that the chance of impact must be so low that it becomes ruled out by many leading scientists, but never really a 0.00000000...% chance. If collision was deemed “certain” as of today, world governments would be actively preparing, and defenses will likely be deployed in time, especially with the newly established Space Force.
Considering the massive alignment of prophetic signs today signaling the second coming (I will likely make a blog post), this may be one of God’s judgements.
These are merely thoughts. Because I am not God, I have zero idea about the future of Earth.
I would very much like to hear thoughts on this.