Charleston - Marine archaeologists said on Tuesday they believe they have located a sunken Civil War-era ship that a group of African-American slaves in Charleston commandeered to sail to freedom 152 years ago.The remains of the storied sidewheel steamship Planter are likely buried off Cape Romain on the South Carolina coast, where the vessel later sank in 1876, the National Oceanic and Atmospheric Administration (NOAA) said. Magnetometer surveys found a large amount of iron signalling a sunken ship about a half mile offshore and buried under 10 to 15 feet of sand and water.Tuesday marked the anniversary of its famous voyage on 13 May 1862.On that day, Robert Smalls, a slave who worked as a deckhand on the Planter during the Civil War, disguised himself as the ship's captain. With his wife, children and other African-American slave families onboard, Smalls sailed the ship past Confederate sentries in Charleston Harbour and out to the Union blockade at sea.Lauded as a hero, he went on to have a notable career in the Union navy as a free man and later served five terms as a congressman for South Carolina."This story has had a lot of resonance, particularly in the Lowcountry communities, places where people still know Robert Smalls' story", said Bruce Terrell, senior archaeologist and maritime historian with NOAA's office of national marine sanctuaries.Expeditions to find the Planter began in 2010 after research of historical documents showed that the ship grounded in a nor'easter in 1876 and sprang a bow plank while trying to salvage another grounded vessel.The ship had offloaded its cotton near Georgetown, South Carolina, but passengers and crew were rescued from it by the crew of the nearby Cape Romain lighthouse, Terrell said.Excavating the Planter is "up in the air", he said, despite interest from museums in artefacts from the ship. Because it is buried, recovery would be expensive and excavation could disturb sensitive loggerhead sea turtle nesting grounds in the Cape Romain national wildlife refuge, Terrell said.A plaque identifying "the last resting place of Robert Smalls' Planter" was dedicated at the refuge's education center on Tuesday.