Atlantis, Lizard People, the hollow Earth and other charming tropes… The Fortean Times was a great source of excellent pulp sf tropes.

I recall reading that Eric Frank Russell was behind the belief in spontaneous human combustion. He was a great sf writer, but would probably not be the best guy to buy a bridge from. Humans, like cows, are soggy and do not catch fire all that easily. The fat ones can probably be got to burn, in much the same way diesel does, but the idea of them suddenly igniting for no good reason except to provide a frisson of horrified fascination is… optimistic.

And yet… people were willing (if put to it by a great snake-oil salesman) to suspend disbelief. One merely has to look at politics! But even in the far more rigorous field of writing sf/fantasy people want to believe. You just… like the snake-oil salesman, have to make it not an unpleasant experience. People picking up a fantasy novel may be tossed out of it because you got a gun or sailboat or worst, horse detail wrong – but that’s because those are real things, which at least some of your readers know are wrong and not possible.

The armadillo-elephant (which defies even the most inventive to actually work out how could cross-breed) will be greeted with delight, and the reader will reach the conclusion it is not really a cross between the two but some magical creature that looks like the result of that. Likewise: Hippogriffs. (an aside, I spent some youthful years firmly under the impression that these hippopotamus x eagle, and was very disappointed to find it merely a horse).

The point is the reader wants to suspend their disbelief. They just don’t want you to use an ordinary horse like an automobile because the writer has never ridden or saddled or fed a horse, and the reader has. In short, the impossible they’ll eagerly accept. The possible, but wrong… they won’t.

The hollow earth and the UFO buried at the South Pole is of course perfectly good story material.