11111...11111 (25-digits)

This number is a prime.

                                                 11111 1666116161 1666111111

     
     1 1 1 1 1
     1 6 6 6 1
     1 6 1 6 1
     1 6 6 6 1
     1 1 1 1 1

Ignoring this square-congruent prime of order 6 minus 1 is devilishly difficult!