The Grand Underground is an expansive labyrinth that takes what was introduced in the original Diamond and Pearl and turns it into something entirely new and brimming with activity.
You can dig down to try and excavate rare items, exchange treasures, or even encounter Pokémon that you wouldn’t normally be able to find in the original games—though it is really tricky to navigate it with how Brilliant Diamond and Shining Pearl mixed things up.
Along with the revamped Secret Base mechanic, the inclusion of 21 unique Pokémon hideaways that house wild Pokémon, and the usual excavation for items, BDSP also split the Grand Underground into six unique sections. All are accessed by dropping below the surface from different overworld locations across Sinnoh.
This means you won’t be able to simply dig down once and run around the entire underground in one go. By splitting things up, the developers are ensuring that the main game and the Grand Underground are intertwined throughout the entire game, rather than just part of your adventures through Sinnoh.
With each of these sections, three of the six can only be accessed by using the Explorer Kit in specific areas of Sinnoh and nowhere else. This includes the central area that only contains a few segments.
If you want to explore the entirety of the Grand Underground or understand exactly which Pokémon Hideways are located within certain segments, here is a full guide for you.
Central Area
The central, and smallest area of the Grand Underground is only accessible by going underground within the borders of Celestic Town.
This doesn’t entirely make sense because the ancient town isn’t actually located at the center of Sinnoh, but it does have a deep connection to Mt. Cornet and the region’s history, so it might be more of a symbolic or coincidental occurrence.
Inside the central area, you will find the following Pokémon Hideways:
- Dazzling Cave
- Icy Cave
- Stargleam Cavern
Top-left Area
Going a bit further north from the centralized area of Sinnoh will allow you to enter the top left portion of the map, which is where almost all of the Ice-type Pokémon-related habitats are going to be popping up.
You can actually only access this portion of the Grand Underground once you exit Mt. Cornet on the way to Snowpoint City, so you can escape the chill and go digging for a bit ahead of getting your seventh Gym Badge. It is also accessible when you are visiting Full Moon island, though that is less likely to come up.
In the top-left area, you will find the following Pokémon Hideways:
- Whiteout Cave
- Icy Cave
- Dazzling Cave
- Spacious Cave
- Glacial Cavern
Bottom-left Area
Similar to how you can only reach the top-left area by digging in the North, the bottom-left area is only accessible when digging near Twinleaf Town, Canalave, City, and the areas nearby. You can even drop down when visiting Ramanas Park in the post-game, too.
In the bottom-left area, you will find the following Pokémon Hideways:
- Spacious Cave
- Grassland Cave
- Fountainspring Cave
- Swampy Cave
- Riverbank Cave
- Sunlit Cavern
Bottom-right Area
Another exclusive area, you can only reach the bottom-right of the Grand Underground by digging beneath the sunny shores of Sunyshore City. And, this area is more expansive than the bottom-right even though it is under just one location… that is one big cave.
In the bottom-right area, you will find the following Pokémon Hideways:
- Spacious Cave
- Rocky Cave
- Fountainspring Cave
- Swampy Cave
- Sandsear Cave
- Bogsunk Cavern
Top-right Area
The top-right of the Grand Underground is the only post-game exclusive area, as it can only be reached by using the Explorer Kit while visiting the Battle Zone and its numerous resorts.
The Pokémon here will be fierce, as you will have reached the maximum requirements to unlock every Pokémon and more by the time you can get there—be ready for a fight even once you go beneath the surface. It is also the only spot you can find the Typhlo Cavern hideaway.
In the top-right area, you will find the following Pokémon Hideways:
- Volcanic Cave
- Dazzling Cave
- Sandsear Cave
- Grassland Cave
- Fountainspring Cave
- Spacious Cave
- Rocky Cave
- Typhlo Cavern
The main area
Curving around the central area and reaching from nearly one end of Sinnoh to the other, the largest area of the Grand Underground is accessible from literally anywhere else not specifically linked to the aforementioned locations.
This means you can dig down on one side of the region and essentially run underground to the other side, though you will still be surfacing back where you entered. Yay Pokémon physics!
In the general area, you will find the following Pokémon Hideways:
- Spacious Cave
- Grassland Cave
- Fountainspring Cave
- Rocky Cave
- Swampy Cave
- Riverbank Cave
- Volcanic Cave
- Sandsear Cave
- Dazzling Cave
- Still-Water Cavern
- Big Bluff Cavern
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