The H.L. Hunley may not hide its secrets for much longer.

Today, scientists at the Warren Lasch Conservation Center will immerse the Civil War-era submarine in a caustic bath of sodium hydroxide and water to begin a long-awaited conservation process that is expected to last about five years.

But within a few months, conservators and archaeologists expect to get their first look at the sub's iron hull - and it may hold clues that finally reveal why the Hunley sank off Charleston 150 years ago.

"We have been waiting for this for years," said conservator Nestor Gonzalez. "Everything we're doing here is to preserve the Hunley as it was and to preserve information recorded on the sub. Now we will finally get to see the hull."

The Hunley conservation project is one of the most complicated in maritime history. This is the only complete ship to ever be conserved in this fashion, and it may be the largest artifact ever preserved with sodium hydroxide.

It will test the capabilities of one of the most advanced maritime conservation facilities in the world. The 40-foot iron submarine is a complex piece of machinery with hundreds of nooks and crannies that will have to be preserved to prevent the sub from rusting away.

The first step will be the most revealing. In the 136 years that the Hunley lay buried beneath the Atlantic seabed, a hard layer of sand and shell formed on the hull.

That concretion, as scientists call it, has protected the Hunley - but it also obscured many details of the hull. The concretion has been left on the sub since its recovery in 2000 for protection.

Gonzalez expects the sodium hydroxide will take only a few months to loosen the hardened sediment enough to allow scientists to scrape it off. Scraping could take months, even with a team of conservators working daily.

Michael Scafuri, Hunley archaeologist, said the concretion has been mapped, photographed and recorded with 3-D imaging to make sure they have every bit of information possible from the Hunley's protective shell.

And now they are ready to see what lies beneath.

"Under that concretion is the possibility of new information about the attack," Scafuri said.

With the shell and sand removed, Scafuri said scientists should learn more about the Hunley's design and operations, and may be able to tell what caused the submarine to sink.

The Hunley disappeared off Sullivan's Island on Feb. 17, 1864, shortly after it sank the Union blockader USS Housatonic by ramming a torpedo into its flank. It was the first successful submarine attack in history, but the mystery of what happened after has endured for a century and a half.

The submarine was buried beneath the Atlantic seabed until 1995, when a dive team funded by Clive Cussler discovered it less than a mile from the Housatonic wreckage.

Removing the concretion is perhaps the most anticipated step in the conservation process, but it is just the beginning.

On Wednesday, scientists prepared the Hunley for its caustic bath and added giant jugs to the sub's tank to help with displacement and reduce the amount of chemicals and water needed by about 8,000 gallons.

Gonzalez said the 76,000 gallon tank will be filled with a caustic mixture that is 99 percent water and 1 percent sodium hydroxide. That solution will start a chemical reaction that will extract salt from the iron hull - salt that soaked into it over a century. The solution will be changed out periodically until all the salt has been removed from the hull.

At that point, it will be safe to display the Hunley without immersing it in water.

This has been the conservation plan from the beginning, and scientists are committed to it. Gonzalez and the other scientists are excited to finally see the hull, but they are also a little worried about soaking a unique artifact in chemicals so dangerous the entire lab has had to be retrofitted for safety. The new safety measures are designed to protect the sub and the scientists, and to allow the lab to remain open for public tours.

"It's like going in to your doctor for a basic procedure," Gonzalez said. "It's safe, but there's always a risk."

But the greater risk, said Senior Conservator Paul Mardikian, is to do nothing. The cold water and mild electrical current that have preserved the sub for the past 14 years cannot protect it forever.

"This is the only way to remove the salt," Gonzalez said.

Mardikian said this is not just the only way to save the sub, it is the last chance to finally answer that nagging question: Why did the Hunley sink?