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Grand Underground Documentation

This page aims to explain the logic that controls Pokémon encounter rates in the Grand Underground. For lists of encounters in the individual areas of the Grand Underground, please see this page.

Links to specific tables below:

• Area Data
• Statue Data
• Pokémon Data
• Digging Item Data

The actual code and data that I looked at to identify how the game is handling encounters can be found as part of this GitHub project, an RNG tool for Grand Underground encounters. The following parts of the project were used to understand this logic:

• run_results.rs contains the logic for calculating type and Pokémon spawn rates.
• statues.rs contains the logic for creating a statue configuration used by run_results.
• StatueEffectRawData.json contains the statue information represented in the Statues table below.
• UgPokemonData.json contains type, size, and spawn weight information for every Pokémon, even those that can't appear in the Underground; only those that can appear are represented in the Pokémon table below.
• Ug_Encount_##.json files contain the specific encounter lists that are used by areas. The name of the json used by each area is shown in the Areas table below.
• UgSpecialPokemon.json contains data for rare Pokémon spawns.
• UgRandMark.json contains data for each of the Grand Underground areas, including type boosts and the Ug_Encount_## json file used by that area.

Order of Operations

The game will use the following logic to determine what Pokémon to spawn. More details will be given on each part of this logic below.

1. The game first determines how many Pokémon will spawn in an area.
2. The game decides whether to spawn a rare Pokémon, and chooses the species of rare Pokémon if there are multiple possibilities.
3. The game performs a calculation to assign each of the 18 types a spawn rate.
4. For each Pokémon that will spawned (based on the number in Step 1), the game selects one of the 18 types, based on the spawn rates of those types calculated in Step 3.
5. Once a type is chosen for a Pokémon, the game will also select a size value to use for the Pokémon. All size values have an equal chance of being used.
6. The game will check the encounter table for the area to find a Pokémon whose type and size values match those generated in Steps 4 and 5. The player must also have advanced far enough along in the story to unlock the Pokémon. If no Pokémon matches the criteria, no Pokémon will be spawned for that encounter slot.
7. If there are multiple Pokémon that match the type and size criteria, the game will choose which Pokémon to use based on the Pokémon's individual spawn weight.

As an example, let's say that the player is in Big Bluff Cavern, the type chosen for the Pokémon is Ground, and the size chosen is 0. Let's assume the player has obtained the National Pokédex and has access to all Pokémon in that area. The following Pokémon meet the criteria: Geodude (10), Sandshrew (15), Cubone (15), Phanpy (15), Larvitar (15), Numel (15), Trapinch (15), Baltoy (15). The numbers in parentheses represent their spawn weights. The rate of any one of these Pokémon will be calculated as follows:

1. Add up all of the spawn weights (115).
2. Find the spawn rate of the Pokémon you want to check and multiply it by 100. (Geodude would be 1000 and any other Pokémon would be 1500.)
3. Divide the result by the total of all the spawn weights (in this case 115). The result is the Pokémon's spawn rate. (For example, Geodude's 1000 divided by 115 is 8.695%, and any other Pokémon's 1500 divided by 115 is 13.043%.)

Number of Spawns

Every area has a minimum and maximum number of spawns assigned to it. When an area is first generated, the number of spawns for the area will be set to the minimum value associated with that area. Following that, the logic will select a random number between 0 and 99, inclusive. If this number is greater than a specific threshold, the number of spawns will be set to the maximum value, instead.

Part of the threshold calculation includes the Secret Base Tile Bonus. This is a number that is based on how many tiles in the player's Secret Base are occupied by placed statues. The Tile Bonus value is as follows:

• 0 tiles: 0 Tile Bonus
• 1-15 tiles: 5 Tile Bonus
• 16-45 tiles: 15 Tile Bonus
• 46-60 tiles: 20 Tile Bonus
• 60+ tiles: 30 Tile Bonus

The threshold is then calculated as (50 - Tile Bonus). For example, if your Secret Base has 50 tiles occupied by Statues, then this threshold would be 50 - 20 = 30. Therefore, if the random number generated is greater than 30, the maximum spawns are used instead of the minimum spawns (which in this case means there's a 70% chance to use the maximum spawns).

Rare Pokémon Calculation

After determining the number of Pokémon to spawn, the game will then check to see if a rare Pokémon should be spawned. This logic proceeds as follows:

1. Generate a random number between 0 and 99.
2. If this value is less than 50, spawn a rare Pokémon.
3. Decide which rare Pokémon to spawn. Every rare Pokémon has a spawn weight attached to it, and the calculation to determine the rate is identical to the Big Bluff Cavern example given above.
4. If a rare Pokémon is spawned, subtract 1 from the number of encounters in the area (meaning the rare spawn replaces a regular spawn).

The rare Pokémon spawn logic is unaffected by type or size calculations and is independent of all other calculations.

Type Calculation

During the process of type calculation, all 18 types will be assigned a spawn rate, and one will be selected based on the spawn rates. There are two factors that influence the spawn rates assigned to types: the area the encounters are being generated in, and the number and identity of Statues the player has in their Secret Base.

Area Rates

The data in the UgRandMark.json file linked above has a type array that contains a value for every type in the game. The ordering is as follows: Normal, Fighting, Flying, Poison, Ground, Rock, Bug, Ghost, Steel, Fire, Water, Grass, Electric, Psychic, Ice, Dragon, Dark, Fairy. The array contains a number that is given to each type. Some areas have higher values for certain types than others, indicating that the area has a natural bias toward those types irrespective of the player's Statue configuration. These values are used as the base values for each type.

Statue Bonuses

The player can have a total of 18 Statues of Pokémon in their Secret Base. Each Statue will provide a boost for the depicted Pokémon's primary and secondary types. The exact bonus varies on a statue-to-statue-basis, and Standard and Color statues have different values.

As of this writing, Bulbapedia claims:

Only the type with the greatest number of statues of the corresponding type will have a boosted effect, which is indicated by the color of the orb placed next to the entrance of the player's base. This "Statue Effect" is increased for every statue of that type that is placed in the player's Secret Base. If the Pokémon has two types, the primary type of the Pokémon has a greater effect than its secondary type in determining which type has a boosted effect. Hence, if only one statue of a dual-typed Pokémon is placed, the boosted type will correspond to the Pokémon's primary type.

Based on my understanding of the code, I believe this to be incorrect. It seems to be the case that Statues' primary types give a larger boost than their secondary types, yes, but it also seems that all Statues are taken into consideration and provide their relevant boosts to any types they reflect. The orb color outside the Secret Base is likely reflective of the type that is provided the highest boost based on the player's Statues.

Once all types have been given a value consisting of the combination of the area's value and the Statue bonus, the values are treated as spawn weights, and summed & divided in the same manner as the other examples above to create the spawn rate of each type. Types are then selected randomly according to these spawn rates and used to generate Pokémon.

Pokémon Calculation

The example I gave earlier for Big Bluff Cavern should be sufficient to indicate how individual Pokémon's spawn rates are calculated. This section provides a few extra data points.

Diglett Bonus

Accumulating 40 points from Diglett and Dugtrio's sparkles activates a 4-minute period during which some bonuses are applied. According to Bulbapedia, the benefits include doubled Shiny probability (which is handled by run_results.rs) and upgrading all Stone Boxes to Gorgeous Stone Boxes. (The documentation here indicates it is actually a 95% chance to get a Gorgeous Box.) Another benefit of the Diglett bonus is to increase the spawn rates of certain Pokémon.

Looking at the code, every Pokémon has a "Rate Up" value associated with it. If a Diglett bonus is active, then when the Pokémon's spawn weight is being added to the sum of weights, the weight will be multiplied by the Rate Up value before it is added to the total. This means that any Pokémon whose Rate Up value is 1 basically has its weight unaffected, and only Pokémon with Rate Up values of 2 or higher will be affected. This seems to only affect starter Pokémon, all of whom have a Rate Up value of 3.

Story Flags

Every Pokémon has six spawn weights associated with it, indicating its spawn weight when different milestones have been reached in the story. A Pokémon with a weight of 0 (shown as "N/A" in the below tables) will not be available at that story flag. Once a Pokémon is added to the list of potential spawns, it is not removed from it. However, the spawn weight can change depending on which story flag is currently being used. The six story flags are as follows:

• Base (available the first time the player can access the Grand Underground)
• Obtain TM96 Strength
• Obtain TM97 Defog
• Obtain the Icicle Badge
• Obtain TM99 Waterfall
• Obtain the National Pokédex

These flags are used in the Pokémon table in this order, as Flag 0, Flag 1, etc.

Levels

The levels of wild Pokémon are also dependent on the amount of progress the player has made, but are instead linked to the number of Badges obtained. See the below table for specifics.

Data Tables

The tables below provide plaintext versions of the data shown in the JSON files linked above.

Area Data

This data contains the name of every area, its ID in the UGRandMark table, the JSON containing its associated encounters, the boosted type(s) provided by that area (with their values), and the minimum and maximum spawns available for that area.

Note that by default, every type has a weight of 50 for an area unless specified otherwise in the table below.

The area with ID 1 appears to be unused or identical to Spacious Cave, aside from its min/max spawns being 3 and 5 instead of 5 and 7.

The area with ID 20 appears to refer to connecting areas, where the player can find Diglett and Dugtrio, and not a specific cavern or cave.

Statue Data

Statues are obtained from Boxes that can be obtained through digging. According to this documentation, boxes have a 50% chance to appear during any digging session. There is a 95% chance to get a Stone Box and a 5% chance to get a Gorgeous Box; during a Diglett Bonus, the rates are reversed.

The game will generate a box by first selecting a type and then selecting a random Pokémon belonging to that type to use for the statue. I am unsure if all types are weighted equally. Within each type, it appears according to the above documentation that all Pokémon are themselves weighted equally, with a spawn weight of 15 used for statues appearing before the National Pokédex is obtained and 5 for statues appearing afterward.

Following the documentation here, and looking at the ordering of data there, it looks like the first instance of each Pokémon species in the list below is the Standard Statue version, and the second instance is the Color Statue version. Color Statues seem to consistently have twice the type bonus values of their Standard counterparts.

No Legendary Pokémon's statues can be found in the Underground. They are listed here to show what their type bonuses are. These are obtained by catching the relevant Legendary.

All statues in the table below (except those of Legendary Pokémon) can be found by digging after obtaining the National Pokédex. Before the National Pokédex is obtained, only the following Pokémon can be found:

Pikachu, Clefairy, Psyduck, Alakazam, Machamp, Tentacruel, Geodude, Rapidash, Gengar, Chansey, Seaking, Mr. Mime, Magikarp, Gyarados, Snorlax, Noctowl, Crobat, Togepi, Marill, Sudowoodo, Aipom, Quagsire, Murkrow, Unown, Girafarig, Steelix, Heracross, Octillery, Mantine, Beautifly, Dustox, Pelipper, Mawile, Medicham, Roselia, Altaria, Whiscash, Milotic, Chimecho, Salamence, and all Pokémon between Turtwig and Rotom (inclusive).

This table contains the ID of every Statue, the Pokémon it reflects, its types, and the bonuses applied to its types.

The Pokémon IDs reflect the Pokémon's National Pokédex number.

Pokémon Data

This table contains the ID of every Pokémon that can be found in the Grand Underground, its types, size, "Rate Up" value, and its weights for each of the six story flags discussed prior.

The Pokémon IDs reflect the Pokémon's National Pokédex number.

Item Data

The information about item spawn rates is sourced from this spreadsheet.

The items that can be found by digging vary depending on the player's story progress. There are two points at which the available items change, or their probabilities change: 1) once the player has completed the events at Spear Pillar, and 2) after the player has obtained the National Pokédex.

Some items only become available at all once one of these milestones has been reached. These items are:

• Odd Keystone (after Spear Pillar)
• Root, Claw, Helix, and Dome Fossils, and the Old Amber (after Spear Pillar)
• Mysterious Shard S and Mysterious Shard L (after obtaining the National Pokédex)

Yes, this does mean that you can get the Kanto and Hoenn Fossil Pokémon before even receiving the National Dex.

I mentioned this above in the Statue Data section, but the source linked above indicates that any given digging session has a 50% chance to have a Box. This appearance rate appears to be separate from the spawn rates used by the 'regular' items shown below.