11111...11111 (25-digits)

This number is a prime.

                                                 11111 1888118881 1888111111

        1 1 1 1 1
        1 8 8 8 1
        1 8 8 8 1
        1 8 8 8 1
        1 1 1 1 1

The only strobogrammatic and tetradic square-congruent prime of order 5. [Ward]