San Juan de Ulúa in Veracruz, where the base level is made of coral. Sometimes bits of wall have worn away, and the coral can be seen underneath. Other times it seems not to have been covered at all. Here, you can the coral seawall (San Juan de Ulúa used to be an island, but filled land has since made it a peninsula) and a bridge over a canal.
Other parts of the site have been used as a prison, including to house future President Benito Juarez. The cells were very damp, and over just a couple hundred years stalactites and stalagmites have formed there. Water torture–constant, inescapable dripping on one’s head–was apparently used to make some of the prisoners crazy.