Grand Underground

The Grand Underground, exclusive to Brilliant Diamond and Shining Pearl, is an expanded version of the Underground from Diamond, Pearl, and Platinum. Like the original Underground, the player can set up a Secret Base, dig for items, and communicate with other players. New to the Grand Underground is the ability to communicate online, as opposed to only locally, and - most prominently - the ability to find wild Pokémon.

Items (Digging)

A variety of normal items can be found by Digging in the underground, including Shards, Fossils, held items, evolution stones, and Plates. The potential items are as follows:

Hideaways

The wild Pokémon that can be captured in the Underground are in Hideaways, large themed areas where multiple Pokémon can spawn. There are multiple Hideaways with the same name throughout the Underground; all of them will have the same Pokémon encounters. My Grand Underground documentation gives an overview of how encounter rates are handled for these Pokémon.

Hideaways are divided into two types: Caves and Caverns, with Caverns being the larger areas. Use the links below to view the Pokémon and items that can be found in each Hideaway.

Caves

Caverns

Rare Spawns

When a hideaway area is entered, there is a 50% chance that one Pokémon in the area will be drawn from a separate set of Rare Spawns. Unlike regular Pokémon, the spawn rates of Rare Spawns are not affected by type bonuses.

Type Bonuses

Every area also has at least one type that will receive a boost for that area. This means that Pokémon who have that type will be more likely to spawn. The boosted types are given in each Hideaway's section. The documentation page linked above provides more details on how these boosts are numerically calculated.

Additionally, the player can create their own type boosts that apply to every Hideaway, alongside the area's native boosts, by placing Statues in their Secret Base. Any Statue placed will provide a type bonus for the type(s) of the Pokémon depicted in the statue. If the Pokémon has two types, their first type gets a larger boost than their second. Color Statues provide a doubled boost compared to Standard Statues.

As far as I can tell by reviewing the code, this is incorrect. All Statues provide boosts to types. It is possible the color of the orb is just reflective of which type currently has the highest boost.

Story Progress

Every Hideaway has a set of Pokémon that will appear anytime the player enters that Hideaway. Some Pokémon, however, require the player to make certain story progress to unlock. The requirements for any Pokémon are shown in the headers in each Hideaway's section.

Levels

The level ranges of the wild Pokémon are tied to the number of badges the player has, as shown below.

Badges	Levels
1	16-20
2	25-29
3	29-33
4	33-37
5	36-40
6	39-43
7	42-46
8	50-55
Defeat Elite Four	58-63

Spacious Cave

Boosted types: Normal

Rare Pokémon

Wild Magnemite have a 5% chance of holding a Metal Coat.
Wild Lickitung have a 5% chance of holding a Lagging Tail.

	Games		Rate
Magnemite	BD	SP	19%
Lickitung	BD	SP	19%
Rhyhorn	BD	SP	19%
Houndoom	BD	SP	19%
Swablu	BD	SP	19%
Absol	BD	SP	5%

Available from the start

Wild Machop have a 5% chance of holding a Focus Band.
Wild Geodude have a 5% chance of holding an Everstone.
Wild Kricketot have a 5% chance of holding a Metronome.
Wild Budew have a 5% chance of holdinga Poison Barb.

	Zubat		Psyduck		Machop		Geodude		Murkrow (Brilliant Diamond)		Bidoof
	Kricketot		Shinx		Budew		Pachirisu		Buneary

After obtaining TM96 Strength

Wild Chingling have a 5% chance of holding a Cleanse Tag.

Golbat

Hoothoot

Bibarel

Chingling

Stunky
(Brilliant Diamond)

Skuntank
(Brilliant Diamond)

After obtaining TM97 Defog

Wild Machoke have a 5% chance of holding a Focus Band.
Wild Graveler have a 5% chance of holding an Everstone.
Wild Wingull have a 50% chance of holding a Pretty Feather.
Wild Munchlax always hold Leftovers.

Machoke

Graveler

Aipom

Wingull

Purugly
(Shining Pearl)

Munchlax

After obtaining the Icicle Badge

Wild Pelipper have a 50% chance of holding a Pretty Feather.

Pelipper

Luxio

Glameow
(Shining Pearl)

After obtaining TM99 Waterfall

Mantyke

After obtaining the National Pokédex

Wild Spearow and Doduo have a 5% chance of holding a Sharp Beak.
Wild Sandshrew and Sandslash have a 5% chance of holding a Grip Claw.
Wild Vulpix have a 5% chance of holding Charcoal.
Wild Oddish and Gloom have a 5% chance of holding an Absorb Bulb.
Wild Venomoth have a 5% chance of holding a Shed Shell.
Wild Slowpoke and Lickitung have a 5% chance of holding a Lagging Tail.
Wild Magnemite have a 5% chance of holding a Metal Coat.
Wild Farfetch'd have a 5% chance of holding a Leek.
Wild Miltank always hold Moomoo Milk.
Wild Zigzagoon have a 50% chance of holding a Potion and a 5% chance of holding a Revive.
Wild Bagon and Shelgon have a 5% chance of holding a Dragon Fang.

Caterpie (Brilliant Diamond)	Weedle (Shining Pearl)	Rattata	Raticate	Spearow	Sandshrew (Shining Pearl)
Sandslash (Shining Pearl)	Vulpix (Shining Pearl)	Oddish	Gloom	Venonat	Venomoth
Mankey	Primeape	Growlithe (Brilliant Diamond)	Slowpoke (Shining Pearl)	Magnemite	Farfetch'd
Doduo	Krabby	Voltorb	Lickitung	Rhyhorn	Rhydon
Tauros	Sentret	Furret	Natu	Teddiursa (Shining Pearl)	Houndoom
Tyrogue	Miltank	Poochyena	Mightyena	Zigzagoon	Skitty
Swablu	Corphish	Crawdaunt	Absol	Bagon (Shining Pearl)	Shelgon (Shining Pearl)

Items

Brilliant Diamond and Shining Pearl Items (Spacious Cave)
Item	Games		Location
Rare Candy	BD	SP	In the southeastern Spacious Cave in the southwestern zone
Rare Candy	BD	SP	In the westernmost Spacious Cave in the east-and-west zone
PP Max	BD	SP	In the middle Spacious Cave in the southeastern zone
Shiny Stone	BD	SP	In the Spacious Cave in the northeastern zone