The Megalodon

Most people consider the most terrifying creature to reign on this planet to be the Great White shark. Its size and sheer ruthlessness can send chills down the spines of any man or beast. The Great White's ancestor, though, is supremely larger and was even more terrifying.
The shark, Carcharodon megalodon, was the greatest hunter the planet has ever seen. Due to the size of the fossilized teeth the creatures length can be found to be anywhere from sixty feet to two hundred feet. Most scientists agree on the sixty foot length; it all depends on where in the mouth the teeth where located. Since the megalodon's skeleton, like all sharks, was constructed of cartilage, it was easily decomposed in a matter of years. The only remains of the creatures are the gigantic teeth that are the size of a man's hand.
Many scientists believe the shark went extinct about two million years ago during the Pliocene epoch. Due to the changing earth climates and the evolution of prey, the Megalodon was thought to have become a creature that couldn't fend for itself. The beast lived in the warmer waters of the oceans and as the climates changed the waters eventually cooled.
There are theories, however, that the Carcharodon megalodon is still alive. Author Steve Alten, whose novels include Meg and The Trench, has pioneered the theory that the shark may still be alive. His theory has the creature slowly moving into the deeper descents of the ocean to find warmth. The shark's metabolism would slow down, and it would require less to eat, so the creature could thrive on the food sources in the deep trenches. Mr. Alten puts it best himself:
We know from fossilized teeth that Megalodon survived the extinction that killed off most of the marine animals 65 & 45 million years ago. In fact, we have teeth that showed Meg's may have been alive as near as 10,000 years ago. What eventually killed them off was the decrease in water temperatures, a result of the last ice age. However, the recent discover of hydrothermal vents (1977) and life forms at the bottom of the deepest parts of the ocean leave the possibility that some Megalodon may have survived by inhabiting these deeper, unexplored waters.
There have been several sightings of gigantic sharks, which could be Megalodon's. The only problem for the Megalodon, and perhaps our hope, is that the several miles of frigid ocean water is keeping the shark trapped. If it were to try and reach the surface, the creature would be killed by the drastic change in temperature.
If the creature still does exist miles under the surface, we can only hope that it never escapes without man's help.

P/S:that's why i m so interested to marine biology..maybe one day i might found a new species of marine creature