Rev 8:8¶And the second angel sounded, and as it were a great mountain burning with fire was cast into the sea: and the third part of the sea became blood;
Rev 8:9 And the third part of the creatures which were in the sea, and had life, died; and the third part of the ships were destroyed.
Has that happened? NO!
{{But Yes!}} Great mountains burning describes volcanoes, and they both create and destroy. They create new land which did not exist and destroy existing land that has been claimed by human and around which cities are built and destroyed by the volcanoe re-activating. Near coasts, their lava and volcanic ash are 'cast into the sea.'
Before THAT happens, THIS happens:
Rev 8:7¶The first angel sounded, and there followed hail and fire mingled with blood, and they were cast upon the earth: and the third part of trees was burnt up, and all green grass was burnt up.
Did that happen? NO!
{{But Yes!}} If you watch the Weahter Channel, the National Oceanic and Atomospheric Administrations (NOAA) public information broadcast channel, you learn all about atmospheric conditions, causes, and effects. Hail and fire are 'cast upon the earth' and a horrible fire that consumed areas of California, Arizona, New Mexico, Colorado, and Nevada made national news not that long ago, in the 21st Century. The fires appeared to burn up 1/3 of America when the fire locations were plotted and the smoke pattern was viewed.
The sea turning to blood is not going to be a man-made catastrophe {{But, Yes!}}
Since man inhabits the earth and takes its resources to sustain itself, why wouldn't a man-made catastrophe be included among the vial judgments of Revelations? The Chernobyl Atomic Accident happened, creating nuclear waste and contamination throughout eastern Russia. This was the Second Seal Revelation. Chernobyl is in the Book of Revelation. It means 'Wormwood" and is interpreted 'bitter and dry grass.' Ask someone from Ukrania to affirm that.