Walkthrough
After defeating the Elite Four and entering the Post Game, you will finally earn access to Cerulean Cave. This cave is located near Cerulean City, and it's home to the Legendary Pokemon known as Mewtwo. No trainers can be found in Cerulean Cave your first time through it, but loads of incredibly strong Wild Pokemon can be. At the very bottom of Cerulean Cave, as mentioned, is Mewtwo, the strongest Wild Pokemon in the entire game.
Wild Pokemon
First Floor & Basement Floor 1:
Golbat (15%): Levels 51-56
Golduck (15%): Levels 51-56
Graveler (15%): Levels 51-56
Zubat (10%): Levels 51-56
Geodude (10%): Levels 51-56
Lickitung (10%): Levels 51-56
Rhyhorn (10%): Levels 51-56
Ditto (5%): Levels 51-56
Rhydon (5%): Levels 51-56
Psyduck (5%): Levels 51-56
Second Floor:
Golbat (20%): Levels 51-56
Graveler (20%): Levels 51-56
Lickitung (15%): Levels 51-56
Rhyhorn (10%): Levels 51-56
Rhydon (10%): Levels 51-56
Chansey (10%): Levels 51-56
Zubat (5%): Levels 51-56
Geodude (5%): Levels 51-56
Ditto (5%): Levels 51-56
All Floors - Special Sapwns:
Snorlax: Levels 51-56
Waters:
Poliwag (5%): Levels 51-56
Magikarp (5%): Levels 51-56
Poliwhirl (5%): Levels 51-56
Poliwrath (5%): Levels 51-56
Obtainable Items
First Floor:
PP Max: Near Northeast Ladder
Max Revive: In Middle Island Between Ladders
Full Heal: Far East River
Max Repel: Far Northwest Area
Full Restore: Hidden Within Raised Area Near Entrance
Second Floor:
Full Restore: Alongside East Wall
Max Revive: Alongside East Wall
Ultra Balls: South Area (Access From Northeast Stairs On First Floor)
Rare Candies: Near Ultra Balls
Max Lure: North Area
Ultra Balls: Hidden In Sparkling Spot (Randomly Respawns Within Minutes)
Ultra Ball: Hidden In Sparkling Spot (Randomly Respawns Within Minutes)
Golden Nanab Berry: Hidden In Sparkling Spot (Randomly Respawns Within Minutes)
Golden Razz Berry: Hidden In Sparkling Spot (Randomly Respawns Within Minutes)
Ultra Ball: Hidden In Sparkling Spot (Randomly Respawns Within Minutes)
Great Ball: Hidden In Sparkling Spot (Randomly Respawns Within Minutes)
Great Ball: Hidden In Sparkling Spot (Randomly Respawns Within Minutes)
Golden Nanab Berry: Hidden In Sparkling Spot (Randomly Respawns Within Minutes)
Golden Razz Berry: Hidden In Sparkling Spot (Randomly Respawns Within Minutes)
Poke Ball: Hidden In Sparkling Spot (Randomly Respawns Within Minutes)
Master Ball: Hidden In Sparkling Spot (Randomly Respawns Within Minutes)
Helix Fossil: Hidden In Front Of Middle Crystal (Randomly Respawns Within Minutes)
Dome Fossil: Hidden In Front Of Middle Crystal (Randomly Respawns Within Minutes)
Old Amber: Hidden In Front Of Middle Crystal (Randomly Respawns Within Minutes)
Basement Floor 1:
Escape Rope: Near Northeast Crystals
Full Restore: After Ramp In Middle Area
Max Elixir: Near Ramp In Southwest
Mewtwonite X: Defeat Pokemon Trainer Green
Mewtwonite Y: Defeat Pokemon Trainer Green
Poke Balls: On Floor After Green Battle
Max Revive: Hidden Near Northwest Crystal
Helix Fossil: Hidden In Front Of Middle Crystal (Randomly Respawns Within Minutes)
Dome Fossil: Hidden In Front Of Middle Crystal (Randomly Respawns Within Minutes)
Old Amber: Hidden In Front Of Middle Crystal (Randomly Respawns Within Minutes)
PP Max: Hidden At End Of West Waters (Respawns Daily)