The most popular theory, later, was that the afterlife was reaching capacity. After eons of human life and death, playing out among a rising population, the great beyond was finally running out of room. The gatekeepers, the theory went, had become more selective about who got in; those who were denied were stuck in limbo among the mortals. Earth was, for the first time, a place where souls both embodied and disembodied coexisted.

The first person to discover this was 48-year-old Monica Dalton, in the small town of High Springs, Ohio. While on her way out of the house to check the mail one summer Sunday, she…