When they are finished in about a year they hope to have the clues as to why the hand-cranked Hunley sank after becoming the first sub in history to sink an enemy warship.

For more than three months, the sub has been in a conservation tank in North Charleston soaking in chemicals to help loosen the hard sand, sediment and rust clinging to the sub. Conservators on Tuesday started using tools similar to those a dentist might use to gently remove the encrustation.

The Hunley sank off Charleston in 1864 after sinking the Union blockade ship Housatonic. The sub was raised in 2000 and brought to the lab.