Swimming with the walls, temples and watchtower of Tulum in background (Mexico, 1999). Technically speaking (yawn), most archaeologists believe its walls date to 1200-1450. Mayan pilgrims still visit and the place is best avoided between 10am and 4pm as its on the tourist bus circus who come on part-day trips from nearly every hotel in the Yacatan (and none of them even bother to have a swim!)