A "gray rhino" is a highly probable, high impact yet neglected threat: kin to both the elephant in the room and the improbable and unforeseeable black swan. Gray rhinos are not random surprises, but occur after a series of warnings and visible evidence. The bursting of the housing bubble in 2008, the devastating aftermath of Hurricane Katrina and other natural disasters, the new digital technologies that upended the media world, the fall of the Soviet Union...all were evident well in advance.