You’ll gradually unlock more of the Grand Underground the more you play, and it’s worth checking it out every chance you get.
How to Unlock Grand Underground in Pokemon BDSP
The Grand Underground unlocks the same way the regular Underground did in the original games. Speak with the Underground Man in Eterna City, near the Pokemon Center, to get the Explorer Kit. This gives you access to the Grand Underground’s starter areas and lets you dig for treasures, shards, elemental stones, and fossils.
Bring the man some fossils as requested, and you’ll also unlock the Secret Base building.
How to Build a Secret Base in the Grand Underground
He gives you the Digger Drill in exchange for the fossils. Use this on a shining spot in the wall to uncover a potential Secret Base location. Choose to build a base there, and you can kit it out with all manner of pedestals and statues.
Putting a rare statue — the green ones — of a certain Pokemon in your Base increases the chance that Pokemon of that type will spawn in the Grand Underground, e.g. a Vespiquen statue increases the likelihood of Bug types appearing.
It seems these statues correspond with the Pokemon’s first type. For example, I’ve noticed more Fire types spawn after I placed my rare Charizard statue, but no increase in Flying types.
Pokemon BDSP Grand Underground Map
The Grand Underground includes multiple locations themed around different elements. Here’s roughly where each region corresponds to in the overworld:
Bottom left: Twinleaf Town, Canalave City Upper left: Fullmoon Island, Snowpoint City Middle: Celestic Town Bottom Right: Sunnyshore City Upper Right: Battle Zone
So, in other words, it’s close to a 1:1 relationship. Dig in a location, and you’ll end up in its corresponding area in the Grand Underground. More locations unlock as you progress in the story.
Grand Underground Pokemon
It’s worth checking out the Underground frequently and exploring as much as you can, since the Pokemon there come with unique benefits.
Most will know at least one Egg move, which is handy if you don’t feel like taking the time to breed Pokemon, and you can encounter certain rare critters long before you will in the overworld. More Pokemon start appearing in the Grand Underground once you unlock the National Pokedex.
Unlike in the rest of Sinnoh, you can actually see the Pokemon roaming in the Grand Underground. If you enter a Hideaway and don’t see the Pokemon you want, exit the area and return to shuffle which Pokemon appear.
How to Dig in the Grand Underground
The mini-map displays digging spots as yellow blips. Interact with the wall there to start the digging mini-game, where you’ll have a limited number of chances to excavate potentially rare items using a pick and a hammer.
The hammer weakens the wall significantly but reveals more areas under the rocks and dirt. The pick is for precision excavation. It only covers one square but weakens the wall less. When you’ve found an item you want, use the pickaxe instead of the hammer.
You’ll potentially find spheres, items, stones, and fossils for your trouble, and you can use the spheres to upgrade your base with room for more statues.
That’s it for the Grand Underground in Pokemon Brilliant Diamond and Shining Pearl, but make sure to check out our other Pokemon Brilliant Diamond and Shining Pearl guides for more tips and tricks.
