1980 Louisiana State University Press trade paperback, Edwin C. Bearss (Petersburg Campaign, The: The Eastern Front Battles, June - August 1864, Volume 1). The Cairo was a Union gunboat which was sunk Dec 12, 1862, after triggering two Confederate demijohn mines. There was no loss of life when the 512-ton ironclad vessel sunk six fathoms deep to the muddy bottom of the Yazoo River north of Vicksburg, Mississippi