From the May 11, 2014, Wilmington (NC) Star-News "Back Then" by Scott Nunn.
He referred to it as a mine, which it actually was, but back then, they were called torpedoes.
On May 13, 1964, a Civil War mine loaded with 500 pounds of black powder was pronounced harmless shortly after being uncovered by two college students at Fort Fisher. It was buried in the sand at the high tide mark.
The cylindrical object was declared safe after it was discovered that water had mixed with the powder. The Coast Guard stood by as Stanley South, archaeologist with the North Carolina Department of Archives and History, assisted by a demolition crew from Fort Bragg opened it. (I'm sure Mr. South was wondering what this had to do with his job description.)
The cone-shaped mine, approximately, 3 feet in diameter, had cones on each end and was found about 3/4 of a mile south of the fort.
Scott Nunn thinks this may be the mine on display at the Fort Fisher museum. It is the one and the same.
The development of torpedoes (mines) was a major element in the Confederacy's battle against the huge Union fleet. Torpedoes sank quite a few Union ships.