As little as 1.6 million years ago, lived a giant shark with the scientific name Carcharodon megalodon. This shark was as much as 50 feet long and could possibly have weighed as much as 60 tons! This was a huge animal even if some of these numbers are a little exaggerated as some have claimed that the weight is.
Some scientists claim that megalodon is a relative of the Great White while others suggest that it was an evolutionary dead end. Regardless, of which theory you happen to subscribe to there are some folks who claim that the oceans are vast and megalodon still lurks in their depths. Considering the Orca evolved less than 5 million years ago and is still with us so why not a megalodon?
As we are aware a Coelacanth was discovered in 1938 and they were thought extinct for millions of years. Today we are still discovering new and large creatures in the oceans of the world so what's to say that someday they won't find a megalodon someday.
There have been reports by credible eyewitnesses during the last century of a giant shark. These reports were made by fishermen and others who would be aware of the kinds and size of creatures in the seas of their local areas. So one would have to assume if they say they saw a huge shark then perhaps they indeed did. We have no reason to doubt people who know the area of ocean that they frequent.
It's entirely possible that these could be sightings of other sharks. The Basking shark can get to about 40 or 45 feet long. It might have been a Whale shark which can grow as large as 50 feet or more. Perhaps these witnesses are simply seeing a very large Great White shark. Could a Great White be responsible for sightings of Megalodon? Regardless, these people are seeing something of note.
Swimmers in Australia have been seeing what they describe as a "monster" shark swimming just off shore. A local beach had to be closed for 2 days due to the fear of this monster. This happened in February of 2007 and some people are suggesting that this is a large Great White or perhaps a large Tiger shark. Unfortunately, no one has been able to identify the shark yet.
We have yet to explore the entire depths of the ocean, it's entirely possible that we have monster that are still to be discovered. Monsters that perhaps live deep and surface once in a while. These surfacing animals would be the cause of some of the sightings that have happened.