Pokemon Brilliant Diamond and Pokemon Shining Pearl are largely the same games as Pokemon Diamond and Pokemon Pearl - but not completely. One thing that's been improved and expanded on in these Pokemon remakes is the underground system. If you want to unlock the Grand Underground - the new and improved subterranean playing field - here's what you need to know.
GRAND UNDERGROUND - How to unlock it
If you want to unlock the Grand Underground, you're going to need to reach Eterna City while playing Pokemon Brilliant Diamond or Pokemon Shining Pearl.
You should be able to find this on your way to beating the Second Gym Leader in both games. After beating Roark in Oreburgh City, you'll need to head back to Jubilife City and then up through the Ravaged Path, Floaroma Town, and Route 205 to hit the Eterna Forest. From here, you can beat Team Galactic at the Windworks and make your way to Eterna City with ease.
Once you're in Eterna City, you should find an NPC in the house next to the Pokemon Center. He's going to give you the Explorer's Kit when you talk to him - something which effectively unlocks the Grand Underground.
Be prepared though, when you talk to him for the first time, you'll be forced into the Grand Underground for an introduction to the area with Roark. You should be able to resurface by hitting Y once you've had a little run around.
Once you return to the surface, you can talk to the man in the house again and he will inform you that you can head back into the Grand Underground at will - through the internet, a local network, or on your own in solo mode.
In the Grand Underground, the following Pokemon are exclusives:
- Brilliant Diamond
- Shining Pearl