The Sinnoh Underground (Japanese: ちかつうろ, Chikatsu Uro, literally meaning; "Underground Passage") is a underground area introduced in the Pokémon Diamond Version, Pokémon Pearl Version and Pokémon Platinum Version. It contains to be a underground area of the Sinnoh region which the player need the Explorer Kit which you can get it from the Underground Man in Eterna City to gain access to the underground. The underground of the Sinnoh region has a lot of fun of discovering items that can't not easily found in the Sinnoh region which exploring in the underground can be compared of being a archaeologist searching for hidden treasures. Luckily, you have to move more with the touch screen instead of pressing the button on the Nintendo DS system. You can create a hidden base and decorate your hideout with different kinds of decorations you obtain.

Basics

Once entering the Sinnoh underground you can use the buttons to move anywhere around the place. On the top screen of the DS, there will be shining stars seen which gives away a hint that there is a treasure hidden. The player must go to the location where you can tap with your stylus pen once or maybe twice to act like a radar which sees the hidden treasures where you must dig to get them. There are also treasures hidden under the ground but not only that, there are also traps hidden in the underground where you must be careful. Furthermore, there are a lot of sphere's hidden in the underground which the player can trade them for traps or decorations.

Test of the Underground Man

Once you've obtained the Explorer Kit, the Underground Man directly asks you to do a test which you can agree on it or not. If you agree to his tests that he gives you what you must do for him in the Sinnoh Underground and complete one of them, he will give you a reward. The Underground give you these tests and items:

No.	Missions	Rewards
1	Go to the Underground.	Move Trap (↑), Bubble Trap and Leaf Trap.
2	Dig up an object.	Prism Sphere 1, Red Sphere 1 and Blue Sphere 1.
3	Bury a Sphere.	Digger Drill.
4	Create a Secret Base.	Plain Table, Wooden Chair, Small Bookshelf and Buneary Doll.
5	Decorate the Secret Base.	A doll Based on your Starter Pokémon.
1	Obtained the Bronze Flag.	Pretty Gem.
3	Obtained the Silver Flag.	Shiny Gem.
10	Obtained the Gold Flag.	Mystic Gem.
50	Obtained the Platinum Flag.	Glitter Gem.