10000...99999 (85-digits)

This number is a prime.

                                                            10000 0000000000
0000000000 0000000000 0000000099 9999999999 9999999999 9999999999 9999999999

The largest prime less than a googol of form 10^(2k)+ 10^k-1, (k=42). [Loungrides]