One of the most famous mythical creatures, the Loch Ness Monster, has an equally notorious basis theory: that the beast is a living plesiosaur. Sightings of Nessie have often taken the form of a broad-humped and long-necked aquatic creature, which fits the plesiosaur description. Although most experts believe that plesiosaurs went extinct along with most dinosaurs at the end of the Cretaceous Period, there are fringe believers who think populations of the beast may have lingered into modern times. There's certainly no shortage of amateur sightings.