Oldie, but goodie: The SATOR square,
SATOR
AREPO
TENET
OPERA
ROTAS
This is one the most well-known unsolved word puzzles, with a history of at least two millennia. Note the TENET cross at the center.
The number of attempts by scholars to answer questions like the following grows yearly:
Who devised it, when, and why?
How has it been utilized in cryptography, medicine, superstition, religion, magic, mathematics, philology, architecture, art work, theology, …
Why examples and variations of it are found in Europe, the Middle East, Africa, and the Americas?
Great references are given below:
IR Thoughts 