It took two days on the bus for Catalina Cespedes and her husband Teodolo Torres to get from their hometown in Puebla - Santa Monica Cohetzala - to Tijuana. On a bright Sunday in May they went to the beach at Playas de Tijuana. There the wall separating Mexico from the United States plunges down a steep hillside and levels off at the Parque de Amistad, or Friendship Park, before crossing the sand and heading out into the Pacific surf.