hi welcome to the forums!
i read a lot of things that say aliens have a base on the bottom of the ocean somewhere
i read a book as a kid called The Secret World of Og about these kids who find a trap door and find an underground city. i was always thinking about that book as a kid and wanting to find a new world underground
it's 3,959 miles to the center of the earth, seems like some creature could have a place in there somewhere that has never been discovered