Now, if they turn out to be man-made, one scenario is that the Gulf Of Mexico was blocked off and dessicated during the Ice Age. This structure could have been built on the much lowered coast and then went underwater from the Ice Age melt when the 'dam' into the Gulf Of Mexico broke, much like the 'dam' to the Black Sea broke in 5600BC.
A half mile is a lot of subsidence. I'm hoping that it is proven to be man-made, maybe then, the whole region will get the attention that it deserves.
I recall a formation off the north coast of Bimini, Bahamas that looked manmade. Stones in a 'pavement' type pattern. It was thought to be the Lost City.
Accepted conclusion now is a natural formation.