Revelation 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.

When the second trumpet angel of the seventh seal sounded John saw what appeared to be a great mountain burning.¹ In other words, he saw an apocalyptic image² that appeared, as it were,³ like a mountain.⁴ A large meteor or small asteroid striking the ocean from outer space best fits this description.⁵ Others suggest that this image could be volcanic⁶ and there are historical types and foreshadows that involve the movement of mountains.⁷ Some people believe the fiery mountain represents a powerful nation,⁸ but that's not consistent with the overall imagery from the first four plagues of the seventh seal.⁹

The precise meaning of the mountain is uncertain,¹⁰ but the literal effects are clear.¹¹ The burning mountain will bring a worldwide plague upon the sea.¹² A third part of the sea will literally become blood to match the first plague of Egypt,¹³ although some scholars believe the water will simply resemble blood.¹⁴

Revelation 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.

Blood in the oceans will bring death to the third part of creatures which [are] in the sea.¹⁵ The third of all ships will also be destroyed.¹⁶ Some refer to the destruction by fractional shares as divine proportioning.¹⁷ Even though the plague is limited to a fraction of the whole, it will surely cripple the fishing and international shipping industries.¹⁸ This will sorely afflict all of humanity.¹⁹

Doctrine and Covenants Section 61:14 describes the same plague, saying: "I, the Lord, in the beginning blessed the waters; but in the last days, by the mouth of my servant John, I cursed the waters."²⁰ John's curse upon the water includes dangers that exist in and upon the waters in the last days through manmade instruments of war.²¹