What is the Underground in Pokémon Diamond and Pearl?

Pick up the Explorer Kit in Eterna City and Pokémon Diamond, Pearl and Platinum let you drop through the floor of Sinnoh into the Underground, a network of tunnels that mirrors the region above. It is two games in one. Alone, it is a mine: tap the walls to find weak spots, then dig with the touch screen before the wall collapses, hunting fossils, evolution stones, Heart Scales, plates and coloured spheres.

The spheres are its currency and its patience test. Spheres dug up small can be buried back in the tunnel floor, where they slowly grow into larger, more valuable ones, then traded to the hikers who live below for traps, furniture and goods. Fossils go the other way, up to the Oreburgh Museum, which revives them into living Pokémon, the only way to own a Cranidos or Shieldon of your own.

With nearby players, over the DS's local wireless, the Underground becomes a playground. Every player digs a Secret Base into the tunnel wall and decorates it, and the sport of the era was capture the flag: sneak into a friend's base, take their flag, and run it home through a floor scattered with traps they had set for exactly that moment.

The Underground also holds Generation IV's strangest errand. Only greetings exchanged down here count toward waking the Spiritomb sealed in the Hallowed Tower on Route 209, a design that assumed you knew other players in real life. Platinum kept the tunnels exactly as they were.