The Cairo is the sole survivor of the fleet of river gunboats built by the Union during the Civil War with the object of controlling the lower Mississippi River. Designed by Samuel Pook and built by James B. Eads, it saw limited battle and was sunk on the Yazoo River in 1862 by newly developed electronically detonated mines, becoming the first craft ever sunk by this predecessor to torpedo technology. The 175-foot ironclad vessel had 13 guns. The Cairo was sunk within 12 minutes, burying much of its gear and armaments and thus preserving a unique view of the Civil War in the river silt for more than 100 years. The propulsion system is the only known early example of the widely used "western rivers" steamboat engine, characterized by multiple fire-tube boilers with shared steam and mud drums and a two-cylinder noncondensing engine having a small bore, long stroke, and poppet valves. This engine was designed by A. T. Merritt. With a 22-inch bore and 6-foot stroke, it developed about 600 horsepower and drove a sheltered paddlewheel of 22-foot diameter and 15-foot width. The Cairo was raised in 1964 and restored by the National Park Service. It, along with its artifacts, is exhibited at the Vicksburg National Military Park.