It started last year when a group of middle school children on a biology field trip in south-central Minnesota spotted some unusual-looking frogs. One was missing a leg, some had withered arms, others had shrunken eyes. Of the 22 frogs caught that day, 11 were deformed. Their teacher told officials. Reports of strange frogs began to mount: a frog with nine legs; a clubfooted frog; a frog with three eyes, one of them in its throat.
At first, investigators from the Minnesota Pollution Control Agency in St. Paul assumed that the problem was restricted to their state, and the agricultural part...