“The belief is found throughout the Celtic territories that certain Standing Stones, set in motion by the spirits which animate them, sometimes go to drink in river or lake. In Orkney, one such Stone was said to walk from the Circle to the Loch of Stennis regularly on Hogmanay, dip it’s head into the Loch, and return to its old position. The story goes that a sailor once seated himself on the Stone some time before midnight in order to test the truth of the legend, and the next morning his dead body was found half-way between the Circle and the Loch”
From “The Silver Bough Volume 1: Scottish Folklore and Folk-Belief” by F.Marian McNeill, 1957 page 87.