Alone in a cell 23 hours a day for days, or even decades. Today, tens of thousands of Americans are subject to what may be the most extreme prison environment ever designed. Although the effects of isolation are largely unknown, in prisons across the country, convicts are placed in solitary confinement cut off from nearly all human contact. Some say solitary is the only way to handle the worst prisoners, but others claim it amounts to psychological torture more cruel than any physical abuse. From prisoners on the edge, to scientists making finds that are changing our most fundamental notions of ourselves, National Geographic looks at the science of solitary and discovers what it means to be absolutely and completely alone.