This story was originally published by Slate and is reproduced here as part of the Climate Desk collaboration.

Last week, Hurricane Harvey devastated the city of Houston. The scale of the tragedy shocked even the National Weather Service. But perhaps the most shocked set of people is the 40 to 50 percent of Houstonians who live outside Federal Emergency Management Agency’s mapped high-risk flood zones and yet still found their living rooms underwater.

FEMA’s maps represent the government’s best estimates of flood plains. They hold a lot of power: When property isn’t included on FEMA’s “magical map,” its residents aren’t required to buy flood insurance. This tends to impart a false sense of security, says Rob Moore, senior water policy analyst at the Natural Resources Defense Council. “That’s why these flood maps lead to such risky behavior and have such tragic consequences.”