Reporter Gustavo Solis meets law-enforcement experts in San Diego, where dense border fencing hasn’t stopped the constant flow of drugs through dozens of huge underground tunnels, an ever-changing array of boats and the San Ysidro Port of Entry, the busiest land crossing in the world. To read, watch and learn more on this topic, visit thewall.usatoday.com, or text the word “DRUGS” to our chatbot at 408-872-9255. This episode was recorded, edited and produced by Hannah Gaber, with field interviews, sound and editing help from David Wallace. Kaila White was our production manager. Our technical director was Nate Kelly. It was written by Gustavo Solis, Jill Castellano and Nicole Carroll.