>> reporter: lewis lachbd is a geologist and sinkhole expert at new mexico tech. >> as best we can tell, the lateral extent from the -- roughly where that traffic light is and then that way beneath the canal and underneath the northern edge of that mobile home park. >> reporter: that welcome to carlsbad sign -- >> that's right exact lie right. >> reporter: disappear in a second. >> i always try to have my camera with me just in case something happens when i drive by. >> reporter: when the first two sinkholes emerged experts figured out they weren't a natural occurrence, they were manmade made. they opened up above two salt mines. they quickly shut down the mining operation in this third site but lewis land says this place is still a time bomb waiting to implode. >> it not a stable configuration. i think that there is -- we know there's a cavity beneath was because they were mining it out. >> reporter: look at this krask cross section of the ground under this corner of carlsbad. all of the blue you see is in the unstable salt well cavern.