http://www.scientificamerican.com/article/fema-does-not-know-if-750-000-homes-are-flood-prone/"About 20 percent of the program's 5.5 million policies are priced too low to cover the risk of flooding. Many of those structures were built before the program was enacted in the late 1960s. So Congress grandfathered them into the program at reduced prices, believing that insurance premiums would offset rising federal disaster aid."
