Super rare. One of two known. There used to be hundreds but they were targeted as objects of war. Salt was valuable and used as a form of economic exchange and the Union sought them out to destroy them. They would have been all up and down the beaches boiling water down to get to the salt.
The magazine had a wooden floor with copper nails. They went down these tiny twisting steps. The outer chamber had ONE candle chamber. It's pretty much pitch black in the actual chamber. Some of us went in. Men had to be barefoot or in shoes that didn't have cleats that would cause sparks.