Born and built amid gray-cloaked secrecy during the American Civil War, the H.L. Hunley -- the first submarine to sink an enemy ship -- has held tight to its murky mysteries.
The 150th anniversary of the Hunley's daring and dangerous raid will be marked this weekend and Monday, but the overarching question remains: What caused the submarine and its eight-member crew to slip to the bottom of the sea on the moonlit evening of February 17, 1864, after it signaled to shore a success that changed naval warfare.
The Hunley, housed at a laboratory in North Charleston, South Carolina, has yielded its secrets slowly and sparingly, even to researchers armed with the latest in technology.
Was the loss of the Hunley the result of the torpedo's detonation? An unsecured hatch? Or perhaps a lucky enemy shot that blasted a hole in the Confederate vessel's viewing port?
And why were the crew's remarkably preserved remains found at their stations, rather than jammed together near an escape hatch?
These and other questions continue to enthrall scientists and historians as the sesquicentennial is observed with tours and events in the Charleston area.
Unmasking the Hunley's secrets
There is hope that some additional clues may emerge soon.
The Hunley Project, a consortium of researchers, scientists and state and federal agencies, this year begins a conservation phase that might add an important piece to the puzzle of what happened to the submarine. A chemical bath will peel away the final layer of sediment that covers the exterior of the well-constructed hull and the Hunley's interior.
"You are going to be blown away. You are going to look at the face of the submarine for the first time," says Paul Mardikian, the project's senior conservator.
Already, the Hunley impresses visitors who gaze down to a 90,000-gallon freshwater conservation tank. Dive planes and remnants of other submarine components, including ballast tanks, are evidence of the innovation and care of the sub's designers and builders.
Patrons at the Warren Lasch Conservation Center also see the encrusted sediment, known as concretion -- a mix of sand and remains of sea life -- that Mardikian likens to a "black box."
By removing the material, he says, researchers will be able to do more precise analysis of holes in the hull and its condition, the Hunley's speed and performance in the Atlantic Ocean and whether gunfire from the USS Housatonic, its target, contributed to the submarine's demise.
"If the submarine was hit by a bullet, you should be able to see that in the metal," says the conservator.
By combining new findings with previous study, including that of the remains of the crew, experts believe they will be able to tell the complete story of what happened to the Hunley, which was brought to the surface amid much fanfare in August 2000.
"I am confident this is all going to fall into place," says Mardikian.
But don't be surprised if everything falling into place won't result in a "smoking gun" that points to a single cause.
"There may be several things (factors) happening at the same time," according to Mardikian.
Archaeologist Michael Scafuri says the team is trying to ascertain the truth of what happened that chilly night a few miles offshore from Charleston. But there are no guarantees.
"It is like detective work -- with a really cold case."
'Curious' submarine a danger to its crews, too
The cold case begins in Mobile, Alabama, where the Hunley was built for the Confederate government.
The 40-foot vessel, described as "curious" looking and resembling a whale, had watertight hatches, two short conning towers, sea cocks, pumps and ballast tanks.
But there were shortcomings. There was constant concern about a sufficient oxygen supply for the crew, which limited its dive time. The captain had a difficult time monitoring certain movements.
The Hunley was dependent on the crew hand-turning a crank to power the single propeller. Batteries and a steam-powered engine proved impractical for the submersible.