Taking years to reach and decades to escape from, the center of the Earth is a purgatory-esque realm that afterlights reach after years of sinking through the earth. Mary Hightower calls this fate "succumbing to gravity fatigue." Mary Hightower also postulates that the presence of afterlights at the center of the earth keeps the earth alive and green.
It's believed that the souls at the center of the earth have reached a sort of nirvana state. They are completely and utterly content to wait for the end of the world and whatever comes with it.