The Sator Square is a word square containing a Latin palindrome featuring the words SATOR AREPO TENET OPERA ROTAS written in a square so that they may be read top-to-bottom, bottom-to-top, left-to-right, and right-to-left. The earliest known appearance of the square was found in the ruins of Pompeii which was buried in the ash of Mt. Vesuvius in 79 AD. Its widespread appeal has been characterized as being a meme.
(via Sator Square - Wikipedia, the free encyclopedia)