Introduction

Hidden Grottoes are special locations scattered throughout Johto and Kanto where you can find rare Pokémon with their Hidden Abilities and valuable items. Polished Crystal features 22 Hidden Grottoes, each with unique Pokémon encounters. Some locations will have Pokémon whose level will scale to your badges. You can check the individual tables below to see what would apply!

These grottoes were inspired by the Hidden Grotto feature from Pokémon Black 2 and White 2 (Generation V), but adapted to fit the Pokémon Polished Crystal experience and carefully curated encounter lists!

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Why Hidden Grottoes Matter in Polished Crystal

Hidden Grottoes are one of the most important optional systems in Polished Crystal because they provide early and repeatable access to Hidden Abilities, which can dramatically change how certain Pokémon perform in battle.

For players building optimized teams, Hidden Grottoes offer:

• Earlier access to competitive abilities
• Pokémon that scale to your current progression
• High-value items without relying on RNG-heavy methods

Ignoring Hidden Grottoes often means missing out on some of the strongest utility and synergy options in the game.

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Optimal Hidden Grotto Strategy

To get the most value from Hidden Grottoes:

• Check grottoes after earning a new badge, when level scaling improves
• Prioritize grottoes with high-impact Hidden Abilities early (e.g., weather, immunity, or stat-boosting abilities)
• Bring the Itemfinder to avoid missing hidden item rolls

Because grottoes reset daily, you are also able to change the clock to farm for specific natures or IVs if desired.

Common Hidden Grotto Mistakes

• Ignoring grottoes early assuming they’re post-game content (My first run, oops)
• Skipping hidden item checks without the Itemfinder
• Farming before badge upgrades, resulting in weaker Pokémon (this isn't too much of a mistake as rather an inconvenience but still worth noting)
• Assuming every Pokémon has a Hidden Ability when some do not
• Assuming every Pokémon will hold an item

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How Hidden Grottoes Work

Basic Mechanics

• Daily Reset: Hidden Grottoes reset once per in-game day (can be changed via the in-game clock system)
• Content Types: Each grotto can contain either a Pokémon, a visible item, or a hidden item
• One-Time Use: Once you interact with a grotto (catch the Pokémon or take the item), it becomes empty until the next daily reset
• Badge Scaling: Wild Pokémon levels scale with your badge count using the formula LEVEL_FROM_BADGES + N (where N varies by grotto)
• First Grotto Guarantee: The first Hidden Grotto you encounter always contains a Pokémon

Spawn Probabilities

While Pokémon encounters are the least common outcome, the guaranteed Pokémon in your first grotto ensures early exposure to the system. Over time, Hidden Items make up a significant portion of grotto value, especially for progression and post-game preparation.

When a grotto resets, it randomly determines what appears:

• 20% chance: Pokémon encounter
• 40% chance: Visible item
• 40% chance: Hidden item (Itemfinder is helpful here but you can probably find it without it too)

Hidden Abilities

All Pokémon encountered in Hidden Grottoes have their Hidden Ability. This is the primary reason to hunt in grottoes-many powerful Hidden Abilities are otherwise very difficult to obtain.

Examples of impactful Hidden Abilities include:

• Abilities that provide immunities or weather synergy
• Abilities that boost stats under common conditions
• Utility abilities that improve switching or survivability

Note: Every pokemon will have a defined hidden ability, but for some Pokémon it'll be less viable than one of their other ones, or it'll be the same as their other ones (but still appear as Ability H in the in-game stat screen)

Level Scaling

Grotto Pokémon levels match your current badge tier plus any grotto-specific modifier. The base badge levels are:

• 0 badges: Level 4
• 1 badge: Level 8
• 2 badges: Level 12
• 3 badges: Level 16
• 4 badges: Level 20
• 5 badges: Level 24
• 6 badges: Level 28
• 7 badges: Level 32
• 8 badges: Level 36
• 9 badges: Level 40
• 10 badges: Level 43
• 11 badges: Level 46
• 12 badges: Level 49
• 13 badges: Level 52
• 14 badges: Level 55
• 15 badges: Level 58
• 16 badges: Level 61

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All Grotto Locations

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Pokémon Encounters

Encounter Rates

Each grotto contains exactly 4 Pokémon species with the following distribution:

• Pokémon 1: 37.5% (common)
• Pokémon 2: 37.5% (common)
• Pokémon 3: 20% (uncommon)
• Pokémon 4: 5% (rare)

When a grotto spawns a Pokémon (20% base chance), one of these four species is selected based on these probabilities.

Notable Hidden Ability Pokémon

Some particularly valuable Hidden Ability Pokémon available in grottoes:

Immunity (Poison immunity):

• Gligar - Found in Mt. Silver grottoes

Multiscale (Reduces damage at full HP):

• Dratini - Dragon's Den grotto

Magic Bounce (Reflects status moves):

• Espeon - Requires breeding from grotto Eevee

Regenerator (Restores 1/3 HP when switching out):

• Slowpoke - Multiple locations

Drizzle (Summons rain):

• Politoed - Evolve grotto Poliwhirl

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Item Drops

Visible Items (40% spawn rate)

When a grotto spawns a visible item, it selects from a weighted probability table including:

• Poké Balls (Great Ball, Ultra Ball)
• Potions (Super Potion, Hyper Potion)
• Repels (Super Repel, Max Repel)
• The grotto's unique rare item

Hidden Items (40% spawn rate)

When a grotto spawns a hidden item (requires Itemfinder), it selects from:

• Mulch (common)
• Mushrooms (Tiny Mushroom, Big Mushroom, Balm Mushroom)
• Leaves (Silver Leaf, Gold Leaf)
• Rare Candies
• PP items (PP Up, PP Max)

Rare Items

Each grotto has one unique "rare item" that can appear as part of its visible item pool. These include evolution stones and other valuable items specific to that location.

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Hunting Strategies

Daily Grotto Route

Optimize your grotto hunting by planning an efficient route:

1. Start in Johto - Check early-game grottoes first
2. Use Fly - Minimize travel time between locations
3. Bring Itemfinder - Don't miss hidden items (40% of spawns)
4. Save before interaction - Soft reset if you want a different nature/IVs

Targeting Specific Pokémon

1. Identify your target grotto from the list above
2. Soft reset until the desired Pokémon spawns (20% chance for any Pokémon, then rarity within that)
3. Save before catching if you want to reset for nature/IVs
4. Catch at appropriate level - Higher badges = higher levels = less training needed

Nature Hunting

Hidden Grotto Pokémon have random natures. For competitive Pokémon:

1. Save before entering the grotto area
2. Check if a Pokémon spawned (20% chance)
3. Check the nature in battle
4. Soft reset if nature is unfavorable
5. Repeat until desired nature appears

Shiny Hunting

Note: Grotto Pokémon are explicitly excluded from shiny generation in the ROM code. Hidden Grotto encounters cannot be shiny.

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Sources

All information in this guide is verified against the Polished Crystal ROM source code:

Core Mechanics

• Grotto count (22 total): polishedcrystal/constants/script_constants.asm lines 241-263
• Grotto data table: polishedcrystal/data/events/hidden_grottoes/grottoes.asm lines 1-165
• Initialization logic: polishedcrystal/engine/events/hidden_grottoes.asm lines 15-93

Spawn Probabilities

• Content type distribution (20/40/40): polishedcrystal/engine/events/hidden_grottoes.asm lines 36-44
• Pokémon selection rates (37.5/37.5/20/5): polishedcrystal/data/events/hidden_grottoes/probabilities.asm lines 1-6
• Item probability tables: polishedcrystal/data/events/hidden_grottoes/probabilities.asm lines 8-30

Daily Reset

• Grotto reset function: polishedcrystal/engine/events/hidden_grottoes.asm lines 114-124
• Daily flag system: polishedcrystal/engine/overworld/time.asm lines 129-141
• Daily flags definition: polishedcrystal/data/events/engine_flags.asm line 151

Hidden Abilities

• Grotto Hidden Ability guarantee: polishedcrystal/engine/pokemon/move_mon.asm lines 236-238
• Normal wild Hidden Ability rate (5%): polishedcrystal/engine/pokemon/move_mon.asm lines 240-242

Level Scaling

• Badge-based level formula: polishedcrystal/data/events/hidden_grottoes/grottoes.asm (LEVEL_FROM_BADGES usage)
• Badge level table: polishedcrystal/data/wild/badge_base_levels.asm lines 3-18

Special Mechanics

• First grotto Pokémon guarantee: polishedcrystal/engine/events/hidden_grottoes.asm lines 31-32
• Shiny exclusion: polishedcrystal/engine/pokemon/move_mon.asm lines 267-268
• Grotto empty flag: polishedcrystal/engine/events/hidden_grottoes.asm lines 126-129

Data Extraction

• Extraction script: scripts/extract-grottoes.ts lines 1-363
• Output manifest: public/new/grottoes.json