Mobile, AlabamaIt was one of the greatest unsolved crimes of the 19th century. Now a team of archaeologists is hunting for the smoking gun—the charred wreck of the Clotilda, the last slave ship to reach the United States before the outbreak of the Civil War.

Experts from the National Geographic Society have joined marine archaeologists and divers from Search, Inc., and the Slave Wrecks Project of the Smithsonian's National Museum of African American History and Culture in an intensive survey of a muddy branch of the Mobile River where the vessel is thought to lie.

The Alabama Historical Commission (AHC) sponsored the expedition after a local reporter found what was thought to be the wreck earlier this year, focusing