Mystery Mate

White plays and gives mate in one move. However, there is a mystery in this position which has to be revealed first.

Mystery Mate

SOLUTION

The mystery is that someone has just placed one extra black pawn on the board - there are 9 in total. Also, no matter which one is the added pawn, there always exists a mate in one move.
If the extra pawn was a7 - Qb6
If the extra pawn was b7 - Kc6
If the extra pawn was c4 - Qb4
If the extra pawn was d3 - Qe4
If the extra pawn was e3 - Bxf2
If the extra pawn was f7 - Ke6
If the extra pawn was g6 - Rg4
If the extra pawn was h3 - Rh4