Book excerpt: On the morning of December 12, 1862, the Union gunboat Cairo, nosing her way up the Yazoo River north of Vicksburg, Mississippi, triggered two Confederate demijohn mines. Within minutes the 512-ton ironclad had sunk six fathoms to the muddy bottom with no loss of life -- the first armored war vessel ever downed by an electronically activated mine. A whole new era of naval warfare had begun.In Hardluck Ironclad Edwin Bearss tells how he and two other Civil War historians discovered the Cairo almost a century later -- still intact at the bottom of the Yazoo, her big guns loaded and ready to fire, much of the gear aboard just as it was that December morning when the crew abandoned her -- and how, almost miraculously, she was later salvaged and restored.