Find the decisive blow (difficulty 1485)

12345678 hgfedcba

Black to move

Show solution
12345678 hgfedcba
  1. a7–b6 key move
  2. a3–a4
  3. b3–c3

What makes it

The pawn sets up a forced mate in nine. It calculates 17 plies deep. At 1485, a real test of vision.