Beginning on April 12, 1861, over the course of two days, more than 3,300 shells and cannon balls rained across Charleston Harbor towards Fort Sumter, the first shots fired in the Civil War. Correspondent Anthony Mason visits the fort with bestselling author Erik Larson, whose latest book, "The Demon of Unrest," explores the events leading up to the bombardment and what Larson calls "the single most consequential day in American history."