"In 1966, a Miami boy smuggled three Giant African Land Snails into the country. His grandmother eventually released them into a garden, and in seven years there were more than 18,000 of them. The eradication program took 10 years, according to the USDA."
Recently, a parent donated several of the beasties to a Wisconsin school. The US Department of Agriculture was called in after teachers learned that their latest classroom pets were illegal aliens. Link