From Bulbapedia, the community-driven Pokémon encyclopedia.
The Underground is an optional, but useful area within Pokemon Diamond and Pearl. It can be accessed after getting the Explorer Kit from the Underground Man in Eterna City.
The Underground has six areas, the northwest, southwest, southeast, northeast, large center, and small center. Each can be reached from different areas throughout Sinnoh.
Here are where you can go to access each area of the Underground.
| Underground Area
|| Surface access
|| Snowpoint City, Fullmoon Island, Newmoon Island, Lake Acuity, Acuity Lakefront
||Twinleaf Town, Sandgem Town, Jubilife City, Oreburgh City, Canalave City, Floaroma Town, Lake Verity, Verity Lakefront, Valley Windworks, Fuego Ironworks, Routes 201, 202, 203, 204, 205 (south), 206, 207, 218, 219, 220, 221
||Fight Area, Battle Park, Survival Area, Resort Area, Stark Mountain, Seabreak Path, Flower Paradise, Routes 224, 225, 226, 227, 228, 229, 230
|| Sunyshore City (south), Routes 212 (south), 222
| Small Center
||Mt. Coronet (summit), Spear Pillar
| Large Center
||Hearthome City, Solaceon Town, Veilstone City, Celestic Town, Eterna City, Pastoria City, Sunyshore City (North), Pokémon League, Solaceon Ruins, Iron Island, Spring Path, Sendoff Spring, Turnback Cave, Eterna Forest (exterior), Lake Valor, Valor Lakefront, Routes 205 (north), 208, 209, 210, 211, 212 (north), 213, 214, 215, 216, 217, 223
Within each of the areas, there will be Hikers that can buy or sell goods and traps, represented on the map as the red (sell for spheres), blue (traps), and yellow (goods) dots.
What To Do
In the Underground, there are two main things that you can do: dig for treasures or make a secret base.
Digging for treasure using the hammer
The primary reason of the Underground is digging up various treasures. These treasure are found by tapping on the bottom screen of your DS and looking for a sparkle. When a sparkle appears on a wall, talk to it. This will open the mining mini-game. Each "field" will have one to four items buried on it. Using your hammer and pickaxe, you need to uncover these treasures. However, be careful. Each hit on the wall will cause the wall to become unstable, and at a certain point, collapse. The treasure that can be found include Spheres, shards, Plates, Revives, Heart Scales, evolutionary stones, held items, and Fossils. If you are playing Diamond Version, the fossil that you will encounter is the Skull Fossil. Those playing Pearl Version will find Shield Fossils. After obtaining the National Dex, Helix Fossils, Dome Fossils, Old Ambers, Root Fossils, and Claw Fossils can be obtained in both versions.
Using the Digger Drill, you can make a secret base in the Underground. Much like the Secret Bases of Ruby, Sapphire, and Emerald, you can decorate your secret base. Other people, though a local connection, can visit your secret base. Upgrades can be made to your secret base by capturing flags from other secret bases.
Leaving the Underground
When you are done in the Underground and want to return to the surface, just open your start menu and select leave. This will return you to the surface exactly where you entered the Underground. Also, items in your treasures bag can be moved to your normal bag before ascending. Only secret base decorations and spheres can not be taken to the surface.