From Bulbapedia, the community-driven Pokémon encyclopedia.
The Evolution Cave (Japanese: 進化ホール Evolution Hole) is a hole at the top left corner of the Red Field from Pokémon Pinball.
The ball may enter the Evolution Cave to start the Evolution Mode (which evolves a Pokémon caught by the player), except when Ditto is blocking the cave.
The Evolution Cave is located in the end of a small corridor at the top left corner of the Red Field. Ditto usually blocks the Evolution Cave, except it leaves when all three Evolution Arrows are on. These Arrows are located at the Left Outside Loop (the outer passage at the left side). The player is able to turn each of these Arrows on by throwing the ball through the Left Outside Loop, from bottom to top. However, Ditto blocks the Evolution Cave again if the Catch 'em Mode, Evolution Mode, or Map Mode is currently active.
If the ball enters the Evolution Cave and the player has at least one Pokémon, the player will be required to choose a Pokémon to evolve and start the Evolution Mode. It is possible to choose a Pokémon that doesn't have an evolution in the first place (for instance, it's possible to "evolve" a Raichu or Gyarados), in which case the chosen Pokémon will "evolve" nonetheless but it won't change its appearance.
If the ball enters the Evolution Cave but the player does not have any Pokémon, then the ball will exit without starting Evolution Mode and the cave will remain unblocked.
| Small Ditto
|| Large Ditto
|| Absent Ditto
| Evolution Cave