The Grand Underground is a vast network of tunnels and caves located beneath the Sinnoh region in the Pokémon universe. It serves as a unique area for exploration, allowing players to dig for treasures, encounter wild Pokémon, and create secret bases. The underground is accessible through various entrances found throughout the region. In addition to its exploration features, the Grand Underground also includes a variety of biomes, each hosting different types of Pokémon. Players can connect with others online to visit each other's secret bases, enhancing the social aspect of the game. This area adds depth to the gameplay experience in the Pokémon Diamond, Pokémon Pearl, and Pokémon Platinum games.