COLUMBIA, S.C. — For years, two scientists have been painstakingly cleaning a century and a half of sand, sediment and corrosion from the first submarine in history to sink an enemy warship.

They drain the 75,000-gallon tank of water and chemicals three times a week for several hours at the Confederate sub’s home in North Charleston, and then go to work in full protective gear, bent around nooks and crannies, gingerly chipping the crud off the H.L. Hunley, all for moments like this, when they can show the world something new.

The most recent discovery, made public last week, involves how the sub moved through the water.

Hidden underneath the rock-hard stuff scientists call “concretion” was a sophisticated set of gears and teeth on the crank in the water tube that ran the length of the 40-foot sub. These gears enabled the crew rotating the crank to propel the sub faster by moving water more quickly through the tube, conservator and collections manager Johanna Rivera-Diaz said.

The biggest surprise for Rivera-Diaz? Discovering that some of the men wrapped the crank handle in thin metal tubes covered with cloth to try to prevent blisters.

“You get really concentrated on a specific area working every day. I was finishing the crank system. One day, when I was through, I just stepped back and ‘Wow, this looks amazing,’?” she said.

The Hunley sank a Union blockade ship in November 1864 by ramming it with a torpedo attached to a spar.

A half-century would pass before another sub sank a ship in the World War I era.

The Hunley itself sank to the bottom during its attack, killing all eight men aboard. Some guess the crew was too close to the torpedo and were knocked unconscious when it exploded, or perhaps miscalculated how long their oxygen would last. Scientists hope to resolve the mystery by cleaning the entire interior of the sub over the next several years.

It took one year to remove all the crud from its hull, and nearly two more to clean out the much smaller crew compartment, Rivera-Diaz said.

“It’s tough physically to do this every day. You are wearing special suits and using chemicals with high pH levels,” she said.

The sub itself is only 4 feet in diameter. Eight schoolchildren can barely cram themselves into a replica nearby at the Warren Lasch Conservation Center.

Up next for Rivera-Diaz is cleaning the conning tower. Scientists have determined that it had a lock, but don’t know why. The submarine was too cramped for the men to move around.

The Hunley was raised from the bottom of the ocean in 2000. Scientists have spent 17 years collecting the human remains and restoring the vessel. Their goal is to get it looking as close to possible as it appeared on its mission.

The eight crew members were buried in an elaborate ceremony at a Confederate cemetery in Charleston in 2004. They were the sub’s commander, Lt. George Dixon of Alabama; James Wicks, a North Carolina native living in Florida; Frank Collins of Virginia; Joseph Ridgaway of Maryland; and four foreign-born men about whom less is known. One is still only known as “Miller.”