The megalodon is a ancestor of the great white shark was the ruler of the seas 25 million to 1 million years ago. This gigantic shark is estimated to have measured 40 to 60 ft long. The average length of a great white shark is 15 ft and it's rare to see one near 20 ft long. The megalodon was thought to be long extinct. Two teeth of this shark have been dated are are said to be only 24,000 and 11,000 years old. 11,000 years is a short period of time when compared to the millions of years the animal was known to be alive, so it still could be possible that the species has survived to modern times
There have been a few sightings of giant sharks resembling the great white over the years. In 1918, some crayfish fisher man reported a shark that was about 115 ft long ate their bait. While they may have exaggerated the length, cutting their estimate in half still gives you a great white shark more than 2 times their normal length. They were also familiar with the local sea life and the great white shark so it's unlikely they confused species. There is also a few theories on how such a large shark could remain hidden all this time. Many cryptozoologists believe that the megalodon spends much of is time in very deep waters, perhaps feeding on giant squid like the sperm whale does.