You can get the Explorer Kit by speaking to an NPC in Eterna City called the Underground Man, who lives in a house next door to the Pokémon Center.
There are no prerequisites to obtaining this item, and you can receive it at any time after you first arrive in Eterna City.
Acquiring the Explorer Kit
The Explorer Kit couldn’t be easier to get; simply head into the house to the right of Eterna City’s Pokémon Center.
The elderly guy you see in the middle of the room is the Underground Man, who was allegedly the first person to explore the Sinnoh Underground and now spends his time encouraging younger generations to do so.
Speak to the Underground Man and he will hand you the Explorer Kit with no questions asked.
Explorer Kit Details and Uses
The Explorer Kit is a Key Item that allows you to explore the Underground. This is a vast network of tunnels underneath the Sinnoh Region where you can find special items, hunt for Fossils, and much more.
To use the Explorer Kit, simply select it from your Bag and choose Use.
Local wireless signals will activate (for multiplayer purposes) and you will be prompted to save the game. Then a hole will open up and you will drop down into the cavernous underworld.
The layout of your screens will change as soon as you hit the bottom: all character action will take place on the touch screen, and an Underground map will be displayed on the upper screen.
You can do several things in the Underground, the most useful of which is digging for items in a mining minigame.
Mining you will also let you uncover Spheres: smooth, round minerals of varying colors and sizes.
These Spheres can be used to buy goods from the many Hikers scattered around the Underground’s tunnels.
Over time, you’ll even be able to create and decorate your own Secret Base for others to find.