#2: Fourth Dimensional Objects
The only reason I am mentioning this theory is because I found a rather funny quote on Listverse.com:
Imagine a flat plane on which two-dimensional creatures live. They cannot perceive our third dimension, but we can see them. Say you stick your finger into their plane. From their point of view, an object has suddenly and inexplicably appeared out of nowhere. It also appears to change size and shape depending on exactly what cross-section of your finger is in their plane. When you pull your finger out, the object disappears, and no one believes the two-dimensional creature who saw it because he was drunk and is always making up crazy stories.
Yep, that’s a possibility, and if it can happen to them, it can probably happen to us too. You know, when dealing with fourth-dimensional objects like UFOs. Problem is, there is no proof about an existing world of two-dimensional stick figures, or a fourth-dimensional one.
#1: UFOs Are a Missing Link
If we maintain that UFOs are indeed ultra-modern spaceships used by an advanced race of creatures, it seems more likely that the creatures originate from Earth than a different planet. People who hold on to this belief often use Ockham’s Razor to justify their viewpoint.
While it seems highly unlikely that any creatures thriving on this surface are still unknown to humans, people often combine this with the Hollow Earth Theory and other “cryptid-related” theories, therefore the term “Cryptoterrestrial”.