On my last day in Puerto Rico, I explored Castillo San Cristobal, a fort built by the Spanish to protect San Juan from attack by land. It was built between 1634 and 1790, and then the U.S. added a few concrete additions during World War II. The fort is huge and has a series of tunnels. Some of these tunnels are huge, connecting the different levels and areas of the fort, and were designed to have defensive explosives. Some of the tunnels are really small, and you have to stoop to move through them. If you ever get a chance to visit, try to get one of the ranger guided tours of the tunnels. Those tours besides being very informative, let you go inside some of the really small tunnels in which you normally aren’t allowed.