Stonehenge

As explained by Geoffrey of Monmouth (c. 1100 – c. 1155), Merlin the magician moved Stonehenge from Ireland to England to serve as a memorial for the hundreds of Britons treacherously slain by the Saxons during a truce meeting on Salisbury Plain. In this story, which Geoffrey claimed was based on an older work he had found, King Ambrosium Aurelianus (uncle of King Arthur) wanted to build a memorial for his dead warriors which would last forever, but his builders could think of no way of doing it. Merlin provided the solution: go to Ireland and bring back the one that’s there.

From http://www.voicesfromthedawn.com/stonehenge/