To go to the Underground in Pokemon Brilliant Diamond and Shining Pearl, you need the explorer’s kit. This item will be useful throughout the game, but you have to unlock it first. This item can only be found once you reach Eterna City. This is fairly early in the game, as it’s the second gym leader location.
It’s not too hard to get, you just need to find the right NPC. The Explorer’s Kit in the Switch remake is found right by the Pokémon Center. When you get here, you need to find the Underground Man’s House. The house you’re looking for has a blue door on it. It’s the one with the yellow roof to the right of the Pokemon Center. Go inside and speak to the old man inside.
Once you have the Explorer’s Kit, you can use it pretty much anywhere on the map. You can dig your way down into the Underground whenever you want, going for a bit of exploration. You can use the Kit from the Key Items tab inside your bag, and teleport into the Grand Underground.
Now that you have unlocked more of the game go out and have fun. And if you want to trade and battle, you can do that too. You don’t need the NSO Expansion Pack to access online in Pokémon Brilliant Diamond and Shining Pearl.
And if you’re more of the collector type of Trainer, there’s good news for you too. Nintendo has confirmed upcoming Home support for both the new remakes. That means you can get into the storage expansion offered by Home and store all those Eggs you’re going to be hoarding. It’s a great help, and can’t really be beaten at $16 or so a year.
