Sounds crazy, right?
But there really could be some truth behind it (potentially...), so read on!
Deep sea expeditions have been analysing the ocean floors for decades now, and the presence of an extremely peculiar element seems to be consistent across the ocean bed, so much so that it's even found at the same depth.
The peculiar element?
Iron-60 is an extremely rare isotope of Iron.
Quick side note for anyone who isn't all that familiar with chemistry:
An "Isotope" of X element, has the same number of electrons and protons but a different number of neutrons. In the case of Iron, the most abundant isotope is Iron-56, which as 26 protons and 30 neutrons, which add together to bring the total mass to 56, hence the name Iron-56.
Iron-60, on the other hand, has 34 neutrons and 26 protons, which add to make 60. The vast difference in neutrons and protons make the nuclei of Iron-60 unstable, which leads to radioactive decay.
Iron-60 is thought to only be produced during a supernova, and the consistency of it across the ocean bed indicates that it all appeared at the same time. This finding suggest that the Earth was hit by the the energetic debris of a supernova, likely to have been very closely by.
Infrared image of the remnants of a supernova, otherwise known as a Nebula... Credit to NASA
But what on Earth does this have to do with the Megalodon?
The Megalodon was an extremely massive shark, coming in at an estimated 15-18m in length. A human standing at even 2m is considered very tall (almost 6"6), and the largest known great white shark was only 6m in length.
Somehow though, the Megalodon reached their demise, and while many people suggest that it was a change of habitat and reduction in food that pushed them to extinction, there's some that draw on the presence of Iron-60 to suspect that a supernova could've finished them off.
Intense cosmic rays are formed and released into the cosmos during a supernova. These highly energetic beams of radiation can break atoms apart upon absorption, releasing a shrapnel-like-effect of subatomic particles known as "Muons".
Muons are negatively charged like electrons, only they're 200 times heavier, so they penetrate substances more deeply. They would pierce through the atmosphere, through the skin of a human, and even go right through small mammals, or on a more related note, right through small non-vertebrates like the average fish.
However, in the case of a large mammal such as the Megalodon, they would be absorbed directly by the atoms that make up the Megalodon, causing them to broken apart.
Such events would cause serious problems for the shark, including radiation poisoning, cancer and likely death, while smaller fish would remain unaffected, as the muons would pass right through them, as opposed to being absorbed.
Only a theory, but an interesting one none-the-less.
Thanks for reading!