Stock Photo - Loch Ness monster. Computer artwork of the Loch Ness Monster swimming on the surface of Loch Ness, Scotland ... . Sightings of the monster have occurred at least since the 15th century, with earlier stories dating to the 6th century, but scientific evidence for its existence is lacking. This depiction is based on Elasmosaurus, a species of plesiosaur a prehistoric marine reptile. Sceptics have suggested that the sightings are of unstable masses of drifting peat. However, the monster's alleged existence continues to attract tourists, and the myth endures.