The map now conforms to the shape of the Sinnoh landmass instead of a large square layout. Notably, it excludes Iron Island, Fullmoon Island, and Route 223 onwards.
The map is still split into six disjoint areas like the original Underground, but the sizes and locations of the six areas do not match those of the original Underground.
Areas now include Pokémon Hideaways, and each area now corresponds to a different aesthetic environment.
At least two areas include surfable water. These hideaways are the Swampy Cave and the Lakeside Cave.
The digging minigame returns following closely to its original incarnation with the exception of digging up Pokémon statues, which come from Stone Boxes. There are at least two kinds of Stone Boxes, the common Pretty Stone Boxes and the rare Gorgeous Stone Boxes. Statues acquired from Pretty Stone Boxes are made of normal stone, while statues from Gorgeous Stone Boxes are made of jade. There is a box for each type.
These statues may seemingly depict Shiny Pokémon, due to the Pokémon's names having the shiny symbol next to it. If at least three statues from Gorgeous Stone Boxes are used, the chances of the desired type appearing are even higher.
Secret Bases return and are created using a Digger Drill as before. Like in the original, a player can only have a single base at a time.
Bases can only be decorated with statues and pedestals to place them on. Adding more of a specific type will slightly increase the chances of that type appearing in hideaways. This "Statue Effect" is indicated by the orb place next to the entrance of the player's base. The flag stealing mechanic has also been removed.
Goods and Traps, with the sole exception of the Digger Drill, have been removed.
Spheres no longer have a numerical size and are instead categorized as Small or Large. They can no longer be buried either.
Instead of goods or traps, merchants (who appear as Hikers) in the Grand Underground can exchange treasures and for spheres. Spheres can now be exchanged for pedestals, items, and TMs.
The radar is now triggered by pressing [R] and searches the area around the player's location.
The Grand Underground supports online communications, compared to the original games which only supported local communications.
Diglett and Dugtrio will occasionally pop out and leave pink sparkles that give 1 and 3 points respectively. Accumulating 40 of these points causes "something good to happen" which increases the shiny odds for Pokémon in Hideaways to 1/2048 for a period of 4 minutes. Additionally, all Stone Boxes found while digging during the 4 minute bonus period will be Gorgeous Stone Boxes. A whistle is heard when this period begins and once more when it ends. The bonus and progress gauge is shared between all players connected to the same instance of the Grand Underground. If all connected players leave the Grand Underground, this gauge resets to zero.
Zones
The Grand Underground is organized into seven zones, with each zone containing Pokémon Hideaways.
Western
Eastern
Central
Northwestern
Northeastern
Southwestern
Southeastern
Pokémon Hideaways
This section is incomplete. Please feel free to edit this section to add missing information and complete it. Reason: Missing information on encounters available in Grand Underground.[1]
Pokémon Hideaways (Japanese: ポケモンの隠れ家Pokémon Hideaway) are new places where wild Pokémon can be encountered. Unlike the main overworld, these areas feature symbol encounters. Each of the seven areas contain at least one large Hideaway, though multiple, smaller Hideaways are scattered throughout the map. Some Pokémon available in the game can only be found in Hideaways. All Pokémon in the Hideaways will have at least one Egg Move if applicable, and earning more Badges, becoming Champion, and obtaining the National Pokédex will increase the strength and variety of Pokémon found in the Hideaways. The Grand Underground also unlocks some Egg Moves that are otherwise unobtainable, e.g. Aura Sphere on Squirtle.
Spacious Caves
Grassland Caves
Sunlit Cavern
Fountainspring Caves
Rocky Caves
Big Bluff Cavern
Volcanic Caves
Swampy Caves
Bogsunk Cavern
Dazzling Caves
Stargleam Cavern
Whiteout Caves
Icy Caves
Glacial Cavern
Riverbank Caves
Still-Water Cavern
Sandsear Caves
Typhlo Cavern
Additional Information
There are three types of merchants in the Grand Underground:
Small sphere merchants allow the player to purchase additional Digger Drills, small or medium pedestals, and TMs which change daily.
Large sphere merchants who offer large or extra large pedestals, and a different set of TMs.
An item exchange merchant, who will trade spheres in exchange for items that can be dug out. Additionally, he can also be paid to expand a player's Secret Base.
Some Hikers found in the Grand Underground allow the player to rest, acting as a Pokémon Center.
If the player runs out of digging spots, more will appear once the player enters and exits a Hideaway.
After the Sinnoh Pokedex is completed Mysterious shards will start appearing in the underground.
Small and Large shards can be used at Ramanas Park in order to purchase different slate items.
The level of Pokémon available in the Grand Underground varies, depending on the number of Gym Badges the Player received and their progression in the game.