This extinct shark was the big daddy of them all. More than three times the size of a great white, the megalodon was a ferocious behemoth that used its massive jaw and a set of 7-inch (12 centimeters) teeth to feast on whales and other gigantic prey. In fact, researchers estimate that this species went extinct around the same time that primitive baleen whales – a group that today includes the humongous blue whale – began growing to their modern, larger size. That's because marine scientists believe that the removal of the 59-foot (18-meter), 55-ton (50-metric ton) predators from sea life allowed the whales to flourish and grow larger. Fossils from the gigantic sharks have been found around the world, from waters off Africa to Europe and the Americas [source: Rincon].
Just what caused the megalodon's demise remains a mystery. What we do know is that this fearsome creature hasn't patrolled Earth's waters in roughly 2.6 million years, despite rumors that the beasts are still lurking [sources: Rincon, Prigg]. Whale watchers can breathe easy.