What is the burning mountain cast into the sea?

When the second trumpet sounds, John sees something like a great mountain, burning with fire, cast down into the sea. It is described exactly as it appears, a huge fiery mass thrown into the ocean, and the effects are immediate and severe.

The result of this mountain being cast into the sea is that a third of the sea turns to blood, a third of all the living creatures in the sea die, and a third of the ships on the sea are destroyed. Other Old Testament passages are brought in to confirm this kind of judgment on the sea and its life, such as the Lord contending by fire and devouring the great deep, and the fish of the sea being consumed along with the beasts of the field and the fowls of heaven.

So the burning mountain itself is simply identified as a great, fire engulfed mass that falls from the sky into the sea as part of God's trumpet judgments, bringing devastating destruction to sea life and shipping.