No one knows exactly how many landmines still litter the world, but it's safe to safe: millions, waiting to kill and maim unsuspecting civilians. Clearing them is slow, expensive and dangerous. The founder of Apopo, Bart Weetjens, talks about his extraordinary project: training rats to sniff out land mines. He shows clips of his "hero rats" in action, and previews his work's next phase: teaching them to turn up tuberculosis in the lab.