If the H.L. Hunley has any secrets left, they are about to be exposed.

Tuesday, scientists at the Warren Lasch Conservation Center will begin scraping away the sand and shell that has covered and encased the Civil War submarine's hull for more than a century.

Beneath that crust, which conservators call concretion, archaeologists will finally get to see the real Hunley. For the first time, modern scientists will be able to examine the sub's actual skin.

"Nobody has seen the hull since it left on its mission," said Stephanie Crette, conservation center director.

And the hull, scientists hope, may contain clues that will help solve the mystery of why the Hunley never returned.

The Hunley sank in 1864, shortly after it sank the USS Housatonic. In the 136 years it spent at the bottom of the Atlantic, sand and sediment and shell accumulated on the hull, eventually hardening like a shell around the iron.

In places, the concretion is a quarter-inch thick; in others perhaps as much as a couple of inches. It has masked many features of the first submarine to sink a ship in battle.

"Even now, we don't know how some of the hull plates are connected," said Nestor Gonzalez, associate director of the conservation center. "We have been waiting for this a long time. We will know if there was any damage to the submarine pre-sinking or post-sinking."

Although the concretion has to be removed so the iron hull can be preserved, the bigger draw to scientists is the mysteries this process could uncover.

While the concretion was left on the submarine to help preserve it in the 14 years since it was raised, it has also made it nearly impossible to determine why the Hunley sank.

If there are any clues to why the Hunley disappeared after its only battle, they will be recorded on the hull.

There are a few things scientists will be looking for in particular: bullet damage, separation of hull plates or other, unrecorded holes.

When the Hunley attacked the USS Housatonic on Feb. 17, 1864, the crew of the Union blockader fired dozens of shots at the sub. The sailors later reported they did no damage to the sub, which rammed a torpedo into the side of the sloop, sinking it in minutes.

Scientists may see evidence of that gunfire, or perhaps some other clues. Some speculate that the concussion from the blast could have caused the sub's hull plates to buckle, allowing water inside it.

There could be small, undiscovered holes in the hull that have been masked by the concretion. Right now, scientists know of three holes in the sub - one on the forward conning tower, one on the starboard stern and another on the forward starboard quarter.

Those last two holes are the result of scouring. Before it was completely covered by sand, the Hunley lay on the ocean floor listing to starboard, and the current literally sanded down the sub's right side.

Removing the concretion will allow for a better study of the forward conning tower hole, a breach made prior to the stern hole at least. Remnants of the busted conning tower were found on the floor of the sub years ago.

The work could take up to a year to finish. Tuesday, two conservators - Liisa Nasanen and Virginie Ternisien - will begin work scraping. Soon, Paul Mardikian and Johanna Rivera will join.

Gonzalez said the scientists are using the Conrad Wise Chapman's 1863 painting of the submarine as a guide. The painting, called fanciful before the sub was recovered, has proven to be a very detailed, accurate portrait of the Hunley.

"There are still features in that painting we have not seen," Gonzalez said.

And they are looking for those, and looking out for unexpected finds. Archaeologist Mike Scafuri said he is particularly interested to find proof that the submarine was painted, as some historical accounts suggest.

Last week, Ternisien and Nasanen tested the technique they will use in a couple of test areas. This is not like scraping ice off a windshield. They are using small hammers and dental chisels most of the time, making the work slow and painstaking. Because there is no schematic of the submarine, and the concretion makes X-ray difficult, they are never sure if they are chipping away sediment or have struck on a rivet or bolt.

"Sometimes a lump of concretion will reveal a new feature, sometimes not," Ternisien said. "You have to be really careful."

The conservators will work six hours straight, three days a week. Based on their tests, it looks like they may be able to uncover as much as a strip a foot long and two inches wide in a day. The rest of the time they will be documenting every inch of the hull they uncover, as well as the sediment they remove.

"The concretion includes a lot of information," Nasanen said. It can explain how long the sub was exposed before it was completely buried, and perhaps even how long that took.

The conservators will start on the wrought iron of the main hull, at first avoiding the cast iron end pieces of the sub that archaeologist believe will be hot spots of information. The team wants to get its technique down before moving into the more complicated areas of the hull. And they will leave a thin sheen of concretion, perhaps a millimeter thick, on the hull to make sure they don't damage the sub.

Once the exterior is finished, they will have to deconcrete the interior, which because of its nooks and crannies, will be even more difficult to clean.

In May, scientists immersed the Hunley in a caustic chemical solution of sodium hydroxide that will extract the salt that seeped into the hull over the years. That process could take five to seven years, and can't be completely effective until the concretion is removed.

Scientists had hoped the chemicals would loosen up that concretion, make it a little easier to move. And it has helped, they say, but it's not like it's falling off.

"You can't say it is easy or difficult," Ternisien said.

"But it's exciting," Nasanen said.