The oldest word square on record, the famed SATOR-AREPO-TENET-OPERA-ROTAS square known form at least three widely separated Roman Empire sites, is also a palindromic square; that is, its five words, when read in order in any direction, form a letter-unit palindrome.
"Some Sentential Palindromic Five-Squares,"
Word Ways: Vol. 37
, Article 10.
Available at: http://digitalcommons.butler.edu/wordways/vol37/iss1/10