Hold L + R and press Y, Left, Down, X, B, Y(2), Left at the title screen. A message will appear to confirm correct code entry.
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Classic Pitfall 2: Lost Caverns mini-game
Hold L + R and press Left, Right, Left, Right, Y(3) at the title screen. A message will appear to confirm correct code entry. The "Lost Caverns" option will be unlocked at the "New Game/Load Game" screen.