The megalodon shark - Carcharodon megalodon - was a giant shark estimated to grow up to 50 or 70 feet in length. Some people have suggested it could have reached up to 80 feet, which would make it even bigger than most land-based dinosaurs. The megalodon was a carnivore that would hunt other creatures in the ocean. It probably used its huge tail or flippers to stun its prey. Based on fossils, its teeth alone are some 5 to 8 inches or more long and its jaws around 6 to 10 feet wide. That sort of hunter could give even a whale a run for its money and it's been suggested that whale meat was one of its favourites.
[Ecologically, a living C. megalodon would seem tro fit somewhere between orcas and sperm whales in dietary choices: the bigger Marine Saurian type would be changing its target  prey items as the individual creature gets bigger, but the best information is that the fully-grown adults eat medium-sized whales and larger sharks preferentially. It seems from the information given here, living C. megalodons also cruise the greater depths for bigger squids to supplement their diet-DD]

Officially the megalodon is extinct, however rumours of its continued existence abound. If these turn out to be true then it wouldn't be the first "living fossil" cryptid to be discovered - the most famous example is the coelacanth.
There have been numerous sightings of unusual giant sharks, possibly megalodons. There are also many globsters (weird unidentified marine corpses) that add weight to the idea. However there have been no confirmed megalodon sightings.
The most famous megalodon sightings were collected by the Australian naturalist David Stead. In the early 20th century he collected reports of a gigantic white shark that had terrorised the fishermen of Port Stephens. Since most of these eye witnesses are anonymous their accounts are often dismissed by sceptics as being anecdotal and of little value. However such reports - whilst not proving the existence of the megalodon - make this mystery an area of interest to many cryptozoologists.