In 1946, the people of Clearwater, Florida, started noticing unusual tracks in the sand on the beach. Specifically, tracks apparently belonging to some three-toed animal that would have had to be big enough to easily body slam a human. Over the next 10 years, the tracks were spotted all over Florida. Some people even reported seeing some kind of hulking bird hanging around, although obviously always from an excessively safe distance.
Newspapers breathlessly reported, and actual scientists were brought in to study the tracks. They theorized they could belong to a giant salamander -- and it was Florida, so it's not like that wasn't a possibility -- but the prevailing theory was a giant penguin. That's not as dumb as it sounds: Penguins the size of humans really did exist once, though they've been long extinct and weren't known to visit Florida.
Whatever it was, it had to have been around 2,000 lbs. Then it just ... disappeared, presumably to put the fear of God and megafauna into a new population of beachgoers.
In 1988, however, the "Clearwater Monster" was unmasked, and it turned out to be the only thing shiftier than the Florida wildlife: the Florida man. Then-25-year-old former Air Force mechanic Tony Signorini had moved from Pennsylvania to Florida in the '40s and started working at an auto shop owned by Al Williams, known to the people and especially the authorities of Clearwater to be a master prankster. He once snuck a horse into a jail cell and rigged a weather balloon on top of the fire station to explode on command, so it sounded like a bomb. It was a different time. (The fire chief got him back by ordering Signorini to call Williams and tell him his shop was on fire.)
In 1946, Williams was reading about dinosaur tracks in National Geographic, got an idea, and started hatching a plan with Signorini. In their shop, they crafted a huge pair of plaster feet and started performing Mythbusters-style experiments. When they couldn't get the plaster to sink into sand deep enough, they went to a blacksmith and had the feet filled with lead. Then they bolted a pair of shoes on top of the feet and started taking midnight rides out in a rowboat along Clearwater's beaches and eventually those of other Florida towns.
For the next 40 years, it was their little secret despite the many people who necessarily knew about it, like Signorini's wife, the blacksmith, and even the police chief, which might explain why the sheriff's department said they had been "personally assured that, if a prank, it was one of the most masterful ever perpetrated" in the county. That was true ... just not in the way they thought it was. Signorini couldn't explain the sightings, though -- even if he'd been in a giant bird costume, which he wasn't, there was never anyone on the beach when he and Williams did their thing -- so it's still possible that the whole thing was a big coincidence and those giant Floridian penguins are having the last laugh.