The birth of American spiritualism is usually traced to Hydesville, N.Y., near Rochester, in 1848, when two young sisters named Margaret and Catherine (Kate) Fox, claimed to hear rappings, or knocking noises, in their room at night. So did their parents. They decided that the noises came from a dead spirit and devised a code to communicate with it.
Word soon spread beyond Hydesville, in the part of upstate New York sometimes called "the burned-over district" because of its waves of religious enthusiasm back then. After a public séance in Rochester failed to detect any fraud, P. T. Barnum invited the sisters to stay at his New York City hotel in 1850, and the editor Horace Greeley praised their powers.
This is the first time I've heard of any of this, neat. My first Google hit yielded this page which is full of fun information.