The claustrum is a mysterious brain structure. The word claustrum means “hidden away.” Indeed, the claustrum is a thin, irregular sheet of grey matter, one sheet in each hemisphere of the brain, concealed beneath the inner surface of the neocortex. Viewed face on, it has an irregular outline remarkably reminiscent of the 48 contiguous United States. The thickness varies from a fraction of a millimeter to several millimeters. The claustrum is not flat but curved in a curious way.