Explore KitUnlike the previous regions, the area of Sinnoh has a hidden underground area. This area has a variety of functions, many of which utilise the DS to DS linkup, but it does not require it. To access it you need to obtain the Explore Kit in Eterna City.
With this item in your Key Items Pocket, you will be able to go underground. Where ever you are in Sinnoh sets the part of the Underground you go into, some of which is closed off so you need to enter it at various different places in Sinnoh in order to go everywhere. Compare the pictures as such. The Top Left, Top Right, Bottom Left & Bottom Right specifically are cut off, only accessible from those areas in Sinnoh. Once you leave the Underground, you will return to where you entered it. Each area has items of various rarities up for trade from the hikers

Sinnoh OverworldSinnoh Overworld

To activate the many events including obstacles and Fossil Hunting, all you have to do is follow the shines on the Map on the Top Screen and when you're close to one, just press the Touch Screen and if there are any by you then you will see a Shining Diamond. Just go up to it and you will activate it

The Sinnoh Underground is also vital if you wish to get the Pokémon, Spiritomb


Excavation

The main thing that you do in the Sinnoh Underground is Excavating in the walls for various items. After locating them, this is controlled purely with the touch screen. It gives you a Pick Axe and a Hammer to use to try and break through the rocks in order to find 2 to 4 items. However there is a limit on how much stress you can induce on the wall through this before the wall collapses itself. To make things worse, there a good number of rocks that are just there to take up space that can cause a cave in if you keep hitting them

There are a number of items that are found here including all the evolution stones, shards that are used to trade for evolution stones, the different plates used for Arceus and various items such as Revives and Hard Stones

You can also find all of the fossils here after you get the National Dex; Dome Fossil, Helix Fossil, Old Amber, Claw Fossil & Root Fossil. Also you can get the Armor Fossil & Skull Fossil depending on your version at any time. However unlike in past games, you can have any number of these fossils on you so you can go revive 20 Omanytes or Anoriths if you do so wish.

The other thing you will find is the currency of the Underground; Spheres. There are 5 different kinds of Spheres found in the underground; Reds, Blues, Greens, Prisms & Pales. However, when you keep getting them, they are not combined and there is only way to combine them and that does cause a loss in the Spheres. What you do is drop the highest amount you have on the ground and keep dropping each subsequent piece onto the ground and then pick it up. You will get the original drop amount plus the extra amounts. Below is a table that states the amounts that are given for the Spheres of particular sizes:

Original Size Added Size
1 - 4 +1
5 - 9 +2
10 - 14 +3
15 - 19 +4
20 - 24 +5
24 - 29 +6
30 - 34 +7
35+ +10

There is another way to get Spheres however, in the far corners of the underground, there are traders that will give you large amounts of gems (in quantities of over 70) in exchange for items you have found. However, be careful as some of the items you find you will only get once such as the Arceus plates.


Secret Bases

Secret Bases make a welcome return. Placed within the Underground world in Sinnoh, you can decorate them like in Ruby & Sapphire. To access them, you need to get a Drill from the old man in Eterna City. This Drill is one use so you have to be careful. However you can pick up another one from the hikers for a price

Once you have created your Base, you'll want to kit it out and hidden amongst the underground are a number of Hikers, all of which sell specific items for a certain amount of the Spheres explained above. Each of the hikers sell different things for your Secret Base for a various amount of Spheres, some of which will definitely require you compile them with the above method. Then you simply kit it out when you get inside the Secret Base.

You can also obtain more items with Mr. E Good's Tasks

Secret Bases do have another use however...


Flag Retrieval

Flag Retrieval is a small mini-game that you and a friend can play. Simply put you need to go to your friend's secret base to get a flag. However your friends are capable of placing traps such as strong wind, boulders etc. to stop you from progressing. The first back wins the game.
The Winner manages to get upgrades to their own Secret Base so they can place more items in it.
There are multiple different levels in the Flag Retrieval, the highest being the Platinum level, obtained by capturing 50 flags from your friend's bases. If you have this ranking, you can remove all of the annoying rocks in your Secret Base


Underground Man

Next to the Pokémon Center in Eterna City is the man that gives you the Explore Kit. Once you have the Explore Kit however, he gives you many tasks that you are to do in the Underground. Do the missions of his, and he'll give you many things. Below is a list of his missions;

Task Reward
Go Underground Move Trap (Up), Bubble Trap, Leaf Trap
Dig Up an Item Prism Sphere 1, Red Sphere 1, Blue Sphere 1
Bury a Sphere Digger Drill
Make a Secret Base Plain Table, Wooden Chair, Small Bookshelf, Buneary Doll
Decorate Secret Base Doll of your Starter (Turtwig, Chimchar or Piplup)
Capture 1 Flag Bronze Flag, Base Upgrade (Up to 12 Decorations), Pretty Gem
Capture 3 Flags Silver Flag, Base Upgrade (Up to 15 Decorations), Shiny Gem
Capture 10 Flags Gold Flag, Base Upgrade (10 Rocks Removed), Mystic Gem
Capture 50 Flags Platinum Flag, Base Upgrade (All Rocks Removed), Glitter Gem

Here is a list of the items that you are able to find in the Sinnoh Underground and their rarity within it:

Spheres

Picture Name Effect Rarity
Blue Sphere Used to Barter with - Worth 1 to 10 Common
Blue Sphere Used to Barter with - Worth 11 to 21 Common
Green Sphere Used to Barter with - Worth 1 to 10 Common
Green Sphere Used to Barter with - Worth 11 to 21 Common
Pale Sphere Used to Barter with - Worth 1 to 10 Uncommon
Pale Sphere Used to Barter with - Worth 11 to 21 Uncommon
Prism Sphere Used to Barter with - Worth 1 to 10 Uncommon
Prism Sphere Used to Barter with - Worth 11 to 21 Uncommon
Red Sphere Used to Barter with - Worth 1 to 10 Common
Red Sphere Used to Barter with - Worth 11 to 21 Common

Fossils

Picture Name Effect Rarity
Armor Fossil Revives into Shieldon Uncommon (Pearl Only)
Claw Fossil Revives into Anorith Rare (After National Dex)
Can be found Rotated by 90, 180 or 270 degrees
Dome Fossil Revives into Kabuto Rare (After National Dex)
Helix Fossil Revives into Omanyte Rare (After National Dex)
Can be found Rotated by 90, 180 or 270 degrees
Old Amber Revives into Aerodactyl Rare (After National Dex)
Can be found Rotated by 90 degrees
Root Fossil Revives into Lileep Rare (After National Dex)
Can be found Rotated by 90, 180 or 270 degrees
Skull Fossil Revives into Cranidos Uncommon (Diamond Only)

Plates

Picture Name Effect Rarity
Draco Plate Raises power of Dragon moves Very Rare
Dread Plate Raises power of Dark moves Very Rare
Earth Plate Raises power of Ground moves Very Rare
Fist Plate Raises power of Fighting moves Very Rare
Flame Plate Raises power of Fire moves Very Rare
Icicle Plate Raises power of Ice moves Very Rare
Insect Plate Raises power of Bug moves Very Rare
Iron Plate Raises power of Steel moves Very Rare
Meadow Plate Raises power of Grass moves Very Rare
Mind Plate Raises power of Psychic moves Very Rare
Sky Plate Raises power of Flying moves Very Rare
Splash Plate Raises power of Water moves Very Rare
Spooky Plate Raises power of Ghost moves Very Rare
Stone Plate Raises power of Rock moves Very Rare
Toxic Plate Raises power of Poison moves Very Rare
Zap Plate Raises power of Electric moves Very Rare

Other Items

Picture Name Effect Rarity
Blue Shard A shard of a bigger plate Uncommon
Damp Rock Prolongs number of turns of Rain Dance Very Rare (Before Nat Dex)
Rare (After Nat Dex)
Everstone Prevents user from evolving when equipped Very Rare (Before Nat Dex)
Uncommon (After Nat Dex)
Fire Stone A stone that can be used to evolve Growlithe, Vulpix and Eevee Very Rare (Before Nat Dex)
Uncommon (After Nat Dex)
Green Shard A shard of a bigger plate Uncommon
Hard Stone Any Rock-type move's power is increased by 10% Very Rare (Before Nat Dex)
Uncommon (After Nat Dex)
Heart Scale A rare scale. Sells High Uncommon
Heat Rock Prolongs number of turns of Sunny Day Very Rare (Before Nat Dex)
Rare (After Nat Dex)
Icy Rock Prolongs number of turns of Hail Very Rare (Before Nat Dex)
Rare (After Nat Dex)
Iron Ball Lowers speed of holder, also makes Flying-types and Levitate Ability using Pokémon vulnerable to Ground moves when equipped to them Very Rare
Leaf Stone A stone that can be used to evolve Gloom, Weepinbell, Exeggcute, and Nuzleaf Very Rare (Before Nat Dex)
Uncommon (After Nat Dex)
Can be found Rotated by 90 degrees
Light Clay Prolongs effect of Reflect and Light Screen Very Rare
Max Revive Revives a fainted Pokémon and fully restores it's HP Very Rare
Moon Stone A stone that can be used to evolve Nidorina, Nidorino, Clefairy, Jigglypuff and Skitty Very Rare (Before Nat Dex)
Rare (After Nat Dex)
Can be found Rotated by 90 degrees
Odd KeyStone A vital item to keep the Stone Tower from Collapsing. Used to call Spiritomb Very Rare
Oval Stone Evolves Happiny into Chansey when equipped and leveled at Morning or Day (4:00am - 7:59pm) Rare
Rare Bone A Valuable Bone. Sell for money Very Rare
Can be found Rotated by 90 degrees
Red Shard A shard of a bigger plate Uncommon
Revive Revives a fainted Pokémon and restores half of it's max HP Very Rare
Smooth Rock Prolongs number of turns of Sandstorm Very Rare (Before Nat Dex)
Rare (After Nat Dex)
Star Piece Rare Star Piece. Sells high. Very Rare (Before Nat Dex)
Rare (After Nat Dex)
Sun Stone A stone that can be used to evolve Gloom and Sunkern Very Rare (Before Nat Dex)
Uncommon (After Nat Dex)
Thunderstone A stone that can be used to evolve Pikachu and Eevee Very Rare (Before Nat Dex)
Uncommon (After Nat Dex)
Water Stone A stone that can be used to evolve Poliwhirl, Shellder, Staryu, Eevee and Lombre Very Rare (Before Nat Dex)
Uncommon (After Nat Dex)
Yellow Shard A shard of a bigger plate Uncommon