In 3 Nephi 8, the Prophet-Historian Mormon records that a tremendous cataclysm destroyed a multitude of cities, slew untold numbers of people, and shook the Nephite and Lamanite civilizations to their cores, bringing them to their knees in every describable way. This event consisted of horrific storms that destroyed entire cities, mass fires that burned other cities to the ground, massive earthquakes that tore apart yet other cities, and three days of darkness that covered the land in a literal, palpable darkness that could be felt and which prevented the kindling of any fire and which blocked out the Sun itself. In short, the Nephites and Lamanites got a practice run with the Apocalypse. But what caused it? What actual physical processes did the Lord use to bring about this Great Disaster? Here I attempt to provide an answer to this question that details all the major events and addresses what caused them. Finally, I comment on some important spiritual truths that we can learn from the history of these events.