When I was a kid I lived near the river just north of Daytona Beach. We found an underground coquina vault that was about 12'x12' 100 yards from the river. The historical society of the area researched it and determined it was a holding spot for runaway slaves headed for the Caribbean. The vault remains there unmarked within the property boundary of the current homeowner. There are a lot of Abandoned Buildings/history in that area.
http://www.reddit.com/r/AbandonedPorn/