One was the Civil War's first submarine, the other was the first sub to take down an enemy ship. One sank en route to attack Charleston, South Carolina, the other sank after defending that same Confederate harbor. One rests somewhere along the shifting ocean floor, the other rests in a well-monitored laboratory tank.
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One was the USS Alligator, which sank in April of 1863. The other was the H.L. Hunley, which plunged some ten months later. For all their differences, both Civil War submarines have a rapidly improving science of shipwrecks working in their favor. Advances in that field have helped researchers narrow the search for the missing Alligator and preserve the remains of the recently recovered Hunley.
"It's a good time to be a marine archaeologist," says Michael Overfield of the National Oceanic and Atmospheric Administration.
Since 2004, Overfield has been searching for the Alligator near Cape Hatteras, an area off the coast of North Carolina known as the "Graveyard of the Atlantic" for the abundance of ships it has consumed. Records indicate that's where the Alligator ended its promising but abortive existence.
Engineered by a French immigrant, the Alligator featured several innovative mechanisms, including a system for removing carbon dioxide from the vessel's interior and a chamber through which a diver could leave, plant a mine and return. The Union Navy considered the Alligator for several missions—most notably, a plan to destroy an important railroad bridge over the Appomattox River—but withdrew the submarine from each of them.
In late March of 1863, shortly after its capabilities had been demonstrated for President Abraham Lincoln, the Alligator headed toward a Confederate harbor in Charleston, towed by the USS Sumpter. On April 2, the tandem sailed full speed into a furious storm. "The Alligator was steering wildly and threatening to snap," the Sumpter's captain later wrote to the Secretary of the Navy, Gideon Welles. At around 6 p.m., the commanders agreed to cut the line, and the angry waves swept the submarine's signature green hull out of view.
Using letters and other primary sources, Overfield and his colleagues at the National Marine Sanctuary Program refined the search area to some 625 square nautical miles. From there, the crew had several new and improved tools to aid their mission. "It's almost like the computer industry," says Overfield. "Think about where we were ten years ago. Did we think we'd be where we are today?"
One of Overfield's options was a magnetometer, which surveys the floor for any magnetic signal—particularly useful when searching for an iron ship such as the Alligator. He also used side-scan sonar, which throws down an acoustic signal to create a picture of everything beneath the boat.
Though these tools have been around for decades, they are now much easier to control, he says. Others, however, have really emerged within the past five years.
Overfield has used what's known as an ROV—a remotely operated vehicle—to further investigate a large object picked up by a magnetometer. The device scours the ocean floor and videotapes the desired area, sparing the cost and danger of sending out a diver. When he wished to cover several targets of interest at once, Overfield employed an autonomous underwater vehicle. These archaeological stunt doubles can be programmed to search a particular area and are equipped with their own magnetometers and sonar.
Though Overfield's search for the Alligator continues, these tools have enabled him to dismiss certain areas where he once believed the ship to be. "That's not always a bad thing, to say 'she is not there,'" he says. "It increases the likelihood of finding her on the next mission, and that's what keeps me going."