From Bulbapedia, the community-driven Pokémon encyclopedia.
The Explorer Kit, featured with the bag, helmet and shovel
The Explorer Kit (Japanese: たんけんセット Exploration Set) is used to access the Underground, an area in Pokémon Diamond, Pearl, and Platinum found beneath the entire Sinnoh region. The Explorer Kit can be used anywhere in the Sinnoh region, except for inside buildings and other structures, to instantly access the Underground. The player cannot dig through elevated structures such as bridges.
When selected from the Bag menu, it asks the user to save the game and indicates that DS Wireless Connection will be turned on. It then transports the player from his or her current location into the corresponding spot of the Underground's tunnels. There are six independent sections of the underground labyrinth, each accessible from different places in Sinnoh. In terms of vendors and mining treasures, the areas are equivalent. When participating in multiplayer action in the Underground, players should make sure they are all in the same set of caverns.
In Pokémon Diamond, Pearl, and Platinum, the player receives the Explorer Kit from the Underground Man in Eterna City, in a house next door to the Pokémon Center any time after arriving in town. He also gives a series of tasks, each with various rewards when they are completed, from Digger Drills to goods.
The Explorer Kit includes all the basic tools used when adventuring, mining, and collecting in the Sinnoh Underground. When certain tools are used for the first time, notes describing their functionality are shown to the user.
Hammer - The hammer is used to dig through walls to find spheres and treasures. It makes a bigger impact on the wall than the pickaxe, digging up several tiles at a time; however, this causes the wall to collapse at double the speed. The wall will collapse after 25 hammer hits.
Pickaxe - The pickaxe is also used to dig through walls. It makes a smaller impact on the wall than the hammer, causing fewer tiles to be dug up at a time. This causes the wall to stay strong for longer before collapsing. If anyone else is mining nearby, a pickaxe will appear on the player's screen wherever the other player hits on their screen. The wall will collapse after 49 pickaxe hits.
Map - The map is displayed on the top screen and shows all of the Underground as well as spots to dig into the walls. There are six independent sections of the labyrinth, each accessible from different places in Sinnoh. In terms of vendors and mining treasures, the areas are equivalent. When in multiplayer mode, the map also shows the positioning of the other players in the Underground.
Radar - The radar locates all treasure, spheres, and traps in the Underground. By touching the touch screen, the radar will appear and locate all treasure, spheres, and traps near to the player. If a sparkle is revealed on the floor using the radar, it is either a trap or a sphere; traps have white sparkles, while spheres have gold. If both are hidden in one spot, a trap is found first, then a sphere. If the sparkle is in the wall, it is a bulging section that can be mined.
The Explorer Kit also comes with containers to carry Spheres and Decorations (aka Goods). Spheres are the currency of the Underground. They can be sold to vendors that appear in the Underground. These explorers are stationary and will only rotate in place. Each of the three types accept spheres, and will trade the player either goods, traps, or treasures.
Goods are generally known as the items that can be used in the Underground to decorate the player's Secret Base, such as dolls, chairs, and tables. Goods can also be purchased in the fourth floor of the Veilstone Department Store, or obtained from Mr. Goods after completing certain requirements. The rarity of the goods is determined by the value of the Sphere being asked for. For example, an Iron Beam is worth a Prism Sphere 9 and a Pink Dresser is worth a Green Sphere 54. This concludes that the Pink Dresser has a much higher rarity than the Iron Beam.