HILTON HEAD ISLAND, S.C. (AP) — During the Civil War, the Union Navy loaded aging whaleships with rocks and towed them south from New England to coastal Georgia and South Carolina.

There the boats were sunk to block key waterways and frustrate blockade runners, small boats designed to elude Union ships and deliver cargo to Confederate states.

Known as the stone fleet, the Union ships were laden with large granite and smooth cobble. What's left of them resembles a pile of rocks on the ocean floor.

And many are scattered throughout the Lowcountry.

Aboard a 26-foot aluminum dive boat Friday, a te...