in order to dig that tunnnn, 900 feet that inn that direction they had to remove hundreds of tons of dirt. yet the neighbors say they saw nothing and heard nothing. i asked the mexican officer with us who builtltt? >> the cartel? el chapo? >> reporter: he said he doesn't know. it was abandoned when they got there. >> right at the end of this street, just four blocks that way, is the u.s./mexico border. that right there is the border fence which means that that tunnel was dug right underneath the noses of body u.s. and mexican authorities. following the line of the tunnel above ground, supervisory border patrol agent tom pittman admits that's probably true. customs ananborder protection has robots they insert in the tunnels. this is one of those robots' view inside drainage pipe used to smuggle drugs. because of the tools and debris so often left in these tunnels the robots often snag and get stuck. they tell us there's no technological solution on the horizon either. what they know is that when they find a tunnel they know they have an exit point and there's only one way to find out