Its secrets have been given up slowly, as though something buried for so long did not want to be brought into the daylight.
But steadily they have emerged from the shell of the Civil War submarine – bones, skulls and personal artefacts, all preserved in the mud of the South Carolina seabed.
Nothing prepared the archaeologists for their most recent discovery: a gold coin, slightly bent and inscribed with the words "April 1862/ My Life Preserver/G.E.D." By itself, that single coin represents one of archaeology's more remarkable finds and recalls a romance that flourished amid the savagery of the war.
The coin was discovered by scientists who have raised the HL Hunley, the world's first successful submarine, which sank in February 1864. The gold piece was found by Maria Jacobson, a Danish scientist and senior archaeologist, as she was cleaning mud from the pelvis bone of the Confederate submarine's skipper, Lt George Dixon. "As soon as I touched it through the mud I knew it was the coin. That was a very rare, teary moment for me," said Ms Jacobson."It was that message from the past we're always looking for."
Lt Dixon's $20 gold coin was said to have been given to him by his Alabama sweetheart, Queenie Bennett, when he left Mobile for the Battle of Shiloh in 1862. Lt Dixon was hit by a bullet that struck the coin and was deflected, sparing his life. It is said he kept the coin with him – inscribed and carried in his left pocket – for good luck.
Lt Dixon was later injured and asked to be made skipper of the HL Hunley, a very basic submarine the Confederates thought could save the war.
The submarine was designed to attach a torpedo to a warship using a spear-like device. It would then retreat and the bomb would be detonated by a rope.
In practice the submarine was a disaster. It sank on two missions, killing part of one crew and all of the second. But in the winter of 1864, with the Union blockade threatening to choke the rebellion, the Confederates was decided to use the Hunley again. That night of 17 February the attack was successful. But while the submarine managed to sink the Union's Housatonic, the Hunley was also lost, sinking 30 feet to the seabed, where it lay for 136 years.
The recovery project – led by the Naval Historical Centre in Washington – succeeded in lifting the vessel from the seabed last summer. Since then the submarine's contents have been sifted.
Dr Robert Neyland, the project director, said: "A lot of archaeology is impersonal. If you are dealing with prehistoric people you are looking at their artefacts – you cannot look at their faces. But with the Hunley you are slowly putting the faces to the individuals."
In addition to the remains of the crew, the archaeologists have recovered pocket knives, pencils, a still-knotted scarf and a Navy button. This alone would have made the $20m (£14m) project worthwhile.
But the discovery of the coin – currently in a secret bank vault – has put things on a different level. History has almost come full circle. And if that circle needed closing, it has been done so by Sally Necessary, the great-granddaughter of Queenie Bennett, the woman who fell in love with the blue-eyed lieutenant but who married someone else when he perished.
Mrs Necessary said: "The [find] was very emotional. I always said that if my great-grandmother had given [Lt Dixon] the coin and it had saved his life then he would still have it."