Revelation 8:8 speaks of a mountain cast into the sea as part of the 2nd trumpet judgment.  (see the judgments of revelation entry for more details)  This is a judgment sent by God during the Tribulation period that will destroy 1/3rd of the sea life and ships, and turn 1/3rd of the ocean to blood.  The bible doesn't specify what this mountain is, but there are two theories that each present a possible answer to this dilemma.  The first suggests that this mountain is an asteroid.  The theory doesn't state where this asteroid will impact, but based on overall sea coverage among the 7 oceans of the world, the most likely location is the Atlantic Ocean.  Since the judgments are directly relevant to Israel, this seems to be the most likely outcome.

The second theory suggests that this is in fact NOT an asteroid, but actually some form of super volcano that rises out of the ocean somewhere in the world and is responsible for the destruction spoken of in the prophecy.  One thing of note.  This is NOT the wormwood prophecy.  That is the 3rd trumpet.  The mountain in the sea is only the second.  Nor is this prophecy the Meteor Destruction of America asteroid impact prophecy, which is a conditional prophecy with no specific timeframe and may not even occur at all, and even if it did, it's not necessarily a Tribulation period event and is separate from this prophecy.