This guide shows how to reach the Grand Underground, and how to make leave the Grand Underground in Pokemon Brilliant Diamond & Shining Pearl (BDSP), released on 19th November 2021. To reach the Grand Underground, you will need to get an Explorer Kit.
How to get Explorer Kit
You will get the Explorer Kit in the Underground Man’s House, which is right next to Pokémon Center in Eterna City as shown in the picture below. Enter the house and talk to the Underground Man to receive the Explorer Kit. The Explorer Kit will be in your Bag’s Key Items pocket.
How to reach Grand Underground
Once you have the Explorer Kit, go to your Bag’s Key Items pocket and use the Explorer Kit to enter the Grand Underground. Then you can explore the Grand underground.
How to Leave Grand Underground
Once you are done exploring the Grand Underground, press ‘Y’ and then select ‘Go up’ to exit the Grand Underground and go back to the surface.