One of my teachers believes megalodon is still alive, what should I tell him? It's the usual "Well, you can't prove it's not around" sort of argument
Oh brother... well, he's definitely wrong, because proving that megalodon isn't around anymore is incredibly easy! It's as simple as, "how come we don't see tons and tons of discarded megalodon teeth that date back only a few years, or even a few centuries and millennia, and we only find fossilised teeth?" See, megalodon tooth fossils are very common, and that's because the meg like all (most?) sharks shed their teeth as they grew new ones. If the meg was still alive we would see those giant teeth, even if the sharks themselves were "hidden" then eventually those teeth would wash up ashore after several years. But we never find them! Nor do we find lots of whale or large fish carcasses with massive bite chunks on beaches, or even living whales with battle scars left over from narrowly escaping the jaws of the meg. We could speculate on the giant squid thanks to sperm whales having fighting scars from them, but there's no similar phenomenon for the meg! The government can't hide EVERY tooth and carcass.
Oh and then there's also the more general stuff. "We've only explored 5% of the ocean we don't know what's out there" that mostly applies to the deep ocean where life is scarce. The megalodon lived in the tropical and subtropical regions of the world and was a predator of the topmost layers of the ocean, most likely. It's also theorised to have given birth close to the coast. In the coastal and shallow parts of oceans we would've sooner or later spotted them. "What if it just evolved to live in the deep sea" Such a large, specialised animal that was dependent on coastal waters for breeding? In such a short amount of time, while its prey was going extinct and the climate cooled? Unlikely. The meg ate whales and large fish to support its humongous size, where does one find such prey on the bottom of the ocean, I wonder? One does not. "Maybe it evolved to filter feed" wh- Then that's no longer the megalodon, is it!!???? There are real, actual super interesting and cool filter feeding sharks that need our attention and protection, and if someone's willing to completely ignore them for the sake of a stupid argument about their beloved megadeath murdershark still existing, then that's pretty sad. At that point you can calmly whisper, "it's okay, shhhh, you can let her go. It's okay. You can miss her, but you need to let her go."
Ah, if you're going to attempt to talk to your teacher about this, I do recommend you don't get snarky like I do about this matter, because that tends to get people on the defensive. No one wants to feel like they're being made fun of (very understandable)! I'm just personally very dumbfounded about people who willfully ignore science in favour of going "I wish this cool giant gigamurder killershark was still alive so I'm going to think that way now"