The Grand Underground is a vast subterranean network accessible from various points in Sinnoh using the Explorer Kit (obtained from the Underground Man in Eterna City). It's a treasure trove of activities:
- Digging for Treasures: Use your pickaxe and hammer to dig up valuable items like Spheres (used for trading), Fossils (can be revived into Pokémon), Evolution Stones, and Plates.
- Secret Bases: You can create your own Secret Base and decorate it with various items. You can also visit other players' Secret Bases via local wireless.
- Statues: Place Pokémon Statues in your Secret Base. These statues have a chance to boost the encounter rate of certain Pokémon types in the Underground.
- Wild Pokémon: In Platinum, the Grand Underground features unique Pokémon hideaways where specific Pokémon can be found, some of which are exclusive to the Underground or appear earlier than on the surface.
Spending time in the Grand Underground is an excellent way to gather resources, find rare Pokémon, and customize your Secret Base. It's also a great way to earn money by selling duplicate treasures.