Sheet by Hematite based on data provided by @Kaphotics, and tables matched to areas based on research by /u/Self-ProclaimedKing!
Clarifying notes/explanation:
- There are 20 total tables distinguished in the code, but there only exist 12 unique encounter lists for regular encounters - all of the seven Caverns (large hideaways) share encounter tables with the basic Caves (small hideaways), but they appear to have distinct sets of special Pokémon. Although we don't know with 100% certainty where each table is used, there is a very compelling theory by /u/Self-ProclaimedKing suggesting the most probable corresponding locations: https://www.reddit.com/r/PokeLeaks/comments/qt979w/my_theory_as_to_how_the_grand_underground/ (I really appreciate the well-reasoned answer to this! This is a fantastic reference point for something a lot of people have been wondering!)
- Each table also pulls from a second table of "special Pokémon," and these special Pokémon are not necessarily shared with the same area as the rest of the encounter table! This is why there are more distinct tables than there are basic encounter lists. I have noted these at the top of each table! (To save some time when you're reading, I noted which full encounter tables are the same as others; only the special Pokémon are different in these areas!) Exactly what is "special" about these Pokémon or why they are stored in their own separate list is not currently known, but they do appear to be available as soon as you reach the Grand Underground for the first time!
- The special Pokémon are stored with their encounter rates, so all of those are provided, but I don't know the relative rarities of any of the normal Pokémon yet and wasn't able to find any indication of them in the mined data. (Let me know if you have a lead on this!) All Pokémon listed are available regardless of the statues you have on display, but statues are known to increase the appearance rate of encounters of a specific type; you should be able to check what type is being boosted from inside your Secret Base after decorating it! Nothing is exclusive to this, but it does make things easier to find.
- More Pokémon become available over time as the player progresses in the game: from the start, after obtaining TM96 Strength at the Lost Tower, after obtaining TM97 Defog at the Great Marsh, after obtaining the Icicle Badge at Snowpoint City, after obtaining TM99 Waterfall at Sunyshore City, and after obtaining the National Pokédex. The columns on the right indicate when each Pokémon becomes available!
- Normally, Pokémon in the Grand Underground can end up with any of their Egg moves! These are listed for convenience. However, there is a file in the data listing exceptions to this - the tab "Forbidden Egg Moves" clarifies the edge cases that can't be found at random on spawns here. In version 1.0, the forbidden Egg moves were not ignored, meaning that all of the moves on the Forbidden Egg Moves tab could still be found! It was only in version 1.1 and on that the proper functionality of ignoring these moves was implemented. This was pointed out to me by @Sibuna_Switch!
- The last thing I could find in the data is a set of level ranges - wild Pokémon in the Grand Underground do scale based on the player's progression in the game! There are nine distinct level ranges, and they appear to correspond to the player's Badge count. The level ranges in the data are as follows: [16 to 20] (from the start), [25 to 29] (two Badges), [29 to 33] (three Badges), [33 to 37] (four Badges), [36 to 40] (five Badges), [39 to 43] (six Badges), [42 to 46] (seven Badges), [50 to 55] (eight Badges) and [58 to 63] (postgame).
- Raw data available here: https://mega.nz/folder/WJkgkZLL#YZOQ2gUPU0g-9ztXxmUbiA/folder/vEtABRTJ
Below is a full map of the Grand Underground with biomes as provided by @SHRetro from Twitter! (The color-coding on the encounter table is matched to the way areas are depicted on this map! You can use this to figure out roughly when each encounter table is available.)