The world is filled with mysteries, and even in the modern age, much of the planet remains unexplored. The depths of the oceans and the intricate and extensive cave systems that honeycomb some parts of the Earth are still largely unknown. Thus, it should come as no surprise that when it comes to this terra incognita, people have projected all sorts of ideas. Tales of sunken cities or lost civilizations are just some of the fanciful theories, and those could even be considered tame in comparison to the idea that Earth is flat and/or hollow.