Pokemon underground

Underground (Japanese: ちかつうろ Underground Passage) is an area in Pokémon Diamond, Pearl, and Platinum found beneath the entire Sinnoh region. Players can travel underground at any time using an Explorer Kit, which they can receive in Eterna City. The Underground features multiple uses of the DS's touch-screen, as the main view is on the lower touch-sensitive screen while the upper screen shows a radar map of the entire underground. It also utilizes DS Wireless Communication to allow multiple players, each with their own DS and copy of the game, to interact. The Underground has a different menu than the one in the overworld. Mostly, it shows the different items used.

Spheres are the currency of the underground. The player can bury their spheres to make them more valuable, either by leaving a sphere buried for a long time to make it grow, or by burying spheres of the same color in the same spot to merge them into a larger sphere