What’s a cave — is it really such a complicated question? Speleologists, the scientists who study them, aren’t always in agreement on this question. Some consider any cavity in the Earth’s crust that was formed in a natural way to be a cave. Then would a mole hole be considered a cave? Other speleologists specify that a cave must be large enough for a person to fit inside without the Sun illuminating all of its area. Then what about caves inside glaciers, where the Sun shines even through thick walls? To avoid confusion, speleologists identify caves by their origin.