Megalodon (Otodus megalodon) was an enormous predatory shark that lived roughly 23 to 3.6 million years ago, in oceans worldwide. It's the largest shark — and one of the largest predatory fish — known to have ever existed.
What we actually know
Sharks have skeletons made of cartilage, not bone, which rarely fossilizes well. Almost everything known about megalodon comes from its fossil teeth and a handful of vertebrae, not complete skeletons. That means size estimates are based on scaling up from tooth size using comparisons with living sharks, which is why estimates vary — but many researchers put the largest individuals at somewhere around 15 to 20 meters (50–65 ft) long, roughly three times the length of a modern great white shark.
Teeth built to match
Megalodon teeth are the clearest evidence of its scale — some fossil teeth exceed 17 cm (about 7 inches) on the diagonal, several times the size of a great white shark's tooth. Their serrated, triangular shape suggests megalodon was built to tackle large prey, likely including whales.
Why it disappeared
Megalodon's extinction, around 3.6 million years ago, roughly coincides with cooling oceans and shifts in the distribution of the whale species it likely depended on for food — though the exact cause is still debated among paleontologists.