When they smell a landmine, they stop, sniff the ground and begin to dig. This signal lets the Apopo staff know they have found a mine or some other explosive, which can then be removed.
Rats, according to Apopo, are much faster than men using metal detectors and are not distracted by metal contaminants. They are much cheaper to maintain than dogs and are easily passed between different handlers.
Human vermin plant them, careful rodents help us to dig them up. And, unlike dowsing rods, it actually works.