Steps from the US Mexico border, there's a hidden battle below. Coming down, we went underground with US Customs and Border Protection for a closer look at how they say they're hitting cartels where it hurts. So, we've made it about 20 ft down, but this is where this system really opens up stretching back this way.We followed Agent Lance Lenor through a tunnel that CBP says was designed by smugglers, one of dozens intercepted by his team. When you get down in a tunnel like this one and you shine your light ...