The biggest disasters seem so far out of the range of the normal possibilities of daily life that it’s nearly impossible to even envision the scale of the destruction and upheaval, even for people who have survived one. Huge hurricanes could all but wash major cities away. Earthquakes on the West Coast and even in the center of the country could knock out power for months, make running water a distant memory, and deprive residents of the roofs over their heads. A deadly epidemic, such as the U.S. hasn’t seen in 97 years, could take the lives of tens of thousands. A terrorist could unleash an improvised nuclear device in a major city, killing thousands—an event without historical precedent.

The people who try to keep the nation ready for these doomsday scenarios call them the Maximums of Maximums, or the MOMs. You might call them the mothers of all disasters. The term comes from the Federal Emergency Management Agency, and for the feds, it generally includes a small universe of possibilities: a major hurricane, a major earthquake, or an improvised nuclear device.