You probably know this but for the benifit of our readers,..

I would not be surprised what can be found in that area. The current fort system is a least the fifth fort built on that site. Going back to the 1600s, the French built one, the British built one, Spanish built two and the Americans built one. The tunnel from the Current Fort Barrancas to the Spanish built Water Battery was built by the spanish in the 1790s. The Fort Barrancus/Advanced Redoubt site was also modified during different periods, especially during the Spanish American War. A trench system was built between the Redoubt and Fort Barrancas either before or during the Civil War. It may have been covered at certain points.

David