THE STORY   For the past seven years, Bart Weetjens has been running a unique lab in Tanzania, where he trains rats to sniff out deadly unexploded land mines -- the legacy of countless bloody conflicts. Although dogs have traditionally been used to help humans detect mines, Weetjens realized that rats are lighter, cheaper to maintain and less susceptible to tropical disease. “I’ve always felt a very strong bonding with rodents,” he says. In “Hero Rats,” FRONTLINE/World reporter Alexis Bloom accompanies Weetjens to work in Mozambique to watch his trained rodents in action.