A large
prehistoric shark, probably the largest predatory
fish (bony or cartilaginous) that ever lived. Reached lengths of up to 67 feet.
Died out at the end of the Pleistocene, though some thin evidence indicates megalodon may have lived up until a mere 10,000 years ago. Some people believe it is possible that megalodon survives somewhere today; Steve Alten's book Meg is based on this theory. Most scientists consider this theory to be a bunch of crap.
A famous photograph exists of six paleontologists posing "inside" a set of megalodon jaws which now reside at the Smithsonian.