"They have things that measure the density of the earth, but when you get into those areas it's very sand, so it's very loose soil, so you get a lot of false readings. The only way that you can find those tunnels is sometimes they cave in, but normally it's through information, informants that know where the tunnels are located at, or there's suspicious activity taking place," Vigil added.
"But it requires a lot joint efforts between Mexico and the United States to find the entrance and then also the exit of those tunnels, requires very specific information, and most of it comes through informants."