Pokémon Brilliant Diamond and Shining Pearl are faithful remakes of the original Generation 4 games so it includes a vast Underground. The Underground is slightly different than what you may be used to in the previous installment of the game. This guide shows everything you need to know about the Grand Underground.
How to gain access to the Underground
Progress through the game to Eterna City:
Once in Eterna City make your way to the building located directly to the left of the Pokémon Center:
Speak to the older gentleman in the center of the room and he will give you the Explorer Kit and teach you how to get into the Unground for the first time:
For any other visit to the Underground go into your bag and head to the Key Items portion and ‘Use’ the Explorer Kit.
Leaving the Grand Underground
Press 
on the controller and Select ‘Go Up’ and it will bring you back to the surface where you entered from.
Digging for Fossils and Treasure in the Grand Underground
Once in the Underground check the map in the upper left corner of the screen and look for Yellow Sparkles on the map and head to that location in the Underground:
Click on the wall where the sparkle is showing on the map and it will tell you there is a bulge in the wall and ask you if you want to dig, select ‘Yes’. It will tell you how many items are in the wall by saying ‘X items pinged in the wall’. That X is the number of items you can find in that spot. Use the claw hammer for smaller digging and the Mallet for bigger areas. Using the Mallet will double the weakness of the wall:
Dig out the items until they are fully uncovered. Once the above crack along the top reaches the other side of the screen the dig will be over automatically so choose where to hit carefully. Anything you fully uncover along the way will be awarded to you after the dig.
Items you can find digging in the walls include
- Pretty Stone Boxes
- Gorgeous Stone Boxes
- Evolution Stones
- Gems
- Plates for Ramana Park
- Fossils
- Revives
- Heart Scales
- Plate Shards
- Power-Up Plates
Underground Merchants
In the Underground there are a series of Hikers scattered all around the Grand Underground that when you speak to them will be willing to trade you items for the gems you dig up. The merchants offer things such as TMs, Larger Gems or items for your Secret Base.
Pokémon Living in the Grand Underground
There are a plethora of Pokémon you will only be able to find in the Grand Underground once you have completed your Pokedex and obtained the National Dex. To find these areas to search for Pokémon head into the Grand Underground and look at the map for the biome rooms:
Inside the Biomes you will find Pokémon you may not be able to find on the surface as easily. This area also has them in 3D models instead of random encounters in the grass so be sure to check these biomes after you complete the game:
Secret Bases
Once you try mining Underground for the very first time and receive your very first Pretty Stone Box you will be given a statue to display in your Secret Base. Head back to the older gentleman in the house in Eterna City that gave you the Exploration Kit and he will give you the Digger Drill. Head back down into the Underground and click on any wall you want your secret base to be located at. It will ask if you want to make a Secret Base, select ‘Yes’ and head into the newly made opening. Once you are inside you may place your statues around your base. Placing more around the inside of your Secret Base will increase the amount of that specific statue’s Pokémon type to appear in the Underground.
Diglett’s Effect Underground
As you roam around the Underground you will notice at intersections there are Diglett and Dugtrio randomly and they run away leaving a pink sparkle:
Collect the Pink Sparkle left behind by the Diglett and you will increase the bar on the left under the map and when maxed it explains that ‘Something good may happen’:
What this does is increases the chance of you finding rarer items such as the Gorgeous Stone Boxes which have ‘Shiny’ Pokémon statues. Do note that the buff does not last very long and will expire after just a couple short minutes. Also, leaving the Grand Underground and coming back resets your Diglett count back to 0/40.
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