Could Megalodon Actually Live In The Mariana Trench?
The Mariana Trench is more than 1,500 miles long (roughly the distance from New York City to Dallas, Texas), and the deepest part of it is called Challenger Deep, which is also the deepest point on Earth. The pressure that far down is so enormously great that very few humans have ever ventured there, but one of the few humans who has is none other than Avatar and Titanic director James Cameron. For a documentary called Deepsea Challenge 3D, Cameron piloted a submarine almost seven miles down to the bottom of Challenger Deep, becoming the third person in history to descend that far. He (along with his predecessors Jacques Piccard and Don Walsh, who descended to record depths in 1960) was able to touch down on the ocean floor, which somewhat blows a hole in The Meg's theory that the "bottom" of the Mariana Trench is actually a thermocline: a layer of extremely cold water separating the rest of the ocean from a hidden world of warm water underneath.
That said, there is some interesting science behind the theory presented in the movie. A thermocline is a layer in the ocean at which the water's temperature drops much more rapidly than the water above or below it. There is a thermocline at the base of the ocean's epipelagic zone, or sunlit zone. Once you get to the bottom of the Mariana Trench, the waters are just slightly above freezing (34-39 degrees Fahrenheit) and the atmospheric pressure is around 15,000 pounds per square inch (psi). To put that in perspective, the atmospheric pressure at sea level is about 15 psi, and a car crusher exerts about 2,000 psi.
That's a pretty hostile environment, which is why humans haven't been able to do much exploring at those depths. There are species that have evolved to live that far down, and marine scientists estimate that there could in fact be thousands of undiscovered species lurking in the ocean's depths. However, Megalodon is definitely not one of them, for the simple reason that there's not enough food down there to sustain a 70 foot shark - or even a regular great white shark, for that matter. There have indeed been giants discovered at the bottom of Challenger Deep, but in this case the word "giant" is relative, as these species are giant amoebas - the largest of them being around 4 inches long. There are also sea cucumbers, jellyfish, and other species at these depths, but nothing that would constitute even a small snack for a monster known for feasting upon whales.
So, we know that Megalodon isn't lurking down in the Mariana Trench, but what about elsewhere in the extremely vast ocean? Well, sorry to disappoint (though it should really come as a relief), but we know that Megalodon isn't hiding anywhere else for the simple reason that there's no trace of it. As you can see from the close-up shots of the Megalodon's mouth in The Meg, shark teeth are arranged in rows, and some sharks can have up to 300 teeth in their mouth at any one time. Sharks are constantly shedding and replacing teeth, at an average rate of about one tooth per week, and depending on the species it's possible for sharks to lose tens of thousands of teeth over their lifetime. However, the youngest Megalodon tooth fossil is around 2.6 million years old, so barring the existence of a Megalodon tooth fairy who has secretly been hoarding every tooth lost since then, it's safe to say that this monster has been extinct for a very long time.
Even if the Megalodon tooth fairy were real, we'd still know about the existence of even a single Megalodon because of the impact it would have on the ocean's ecosystem. If a shark of this size were out there preying on whales, giant squid, and other wildlife, it would cut a noticeable swathe through the population. For Megalodon to exist without us noticing, it would need to be a ghost shark - and that's another movie entirely.