The city of Moroni was along a coast, and it is destroyed by the sea. The precise meaning of “sink into the depths of the sea” can be open to debate. Either a massive earthquake caused the land to sink, or the ocean itself rose up. This latter is perhaps the better possibility, as earthquakes are known to cause tsunamis in costal areas (Hugh Nibley. Since Cumorah. Deseret Book, Salt Lake City, 1970, p. 264-5. See also John A. Tvedtnes. “ Historical Parallels to the Destruction at the Time of the Crucifixion.” Journal of Book of Mormon Studies 3:1 1994, for descriptions of the historical destructions from tsunamis).