On April 21, 1934, the U.K. newspaper the Daily Mail published a photo of a mysterious reptile resembling a long-necked dinosaur emerging from the tides of Loch Ness in Scotland. It was purportedly a photo of the Loch Ness Monster taken by London doctor Robert Kenneth Wilson, who said he'd noticed a disturbance in the lake's water while driving by. People believed him because of his social stature, though there were skeptics. But 50 years later, another Nessie enthusiast, Alastair Boyd, rediscovered an overlooked 1975 newspaper clipping claiming the photo was a hoax.
In a twist, it all turned out to be a revenge plot. A couple of years before the photo was published, a man named Marmaduke Wetherell claimed to have found footprints from the monster, but they were later found to have been made with an umbrella stand fashioned out of a hippopotamus foot. He was publicly mocked for the incident in the Daily Mail.
To get back at the paper, Wetherell had his son-in-law and sculptor Christian Spurling build a toy submarine shaped like the creature. Then he had the photo taken, possibly by his son Ian Wetherell, the eventual author of that 1975 article. Wilson, the doctor, had sold the photo to the Daily Mail after being contacted by Marmaduke through a mutual friend.
How do we know all this? Boyd told the press after he interviewed the only surviving character in the story, Spurling, who was nearing death. He verified that it was all a ploy to fool the Daily Mail. It was one that went off swimmingly.
You can read more about the monster's history at the PBS NOVA website.