The Mariana Trench Megalodon
In 2018, the Jason Statham movie The Meg introduced the world to the novel concept of giant Megalodon sharks lurking in the Mariana Trench. The movie depicts the Mariana Trench having a “fake” bottom, behind which these super-sharks have lurked all along, but apart from that novel feature, the conspiracy theories about Megalodons secretly haunting the seas have been around for quite a while — and what better location for them to hide their existence from puny humanity than the deepest pit in the sea?
Unfortunately (or fortunately, depending on your views on massive sharks), this is very unlikely to be true. The Mariana Trench could not even theoretically support a creature as large as the Megalodon, and anyway, the creature used to hang around in fairly shallow, warm waters. But hey, one can always dream, right?