In Sinnoh, there is an underground world to discover. You can dig up treasure there and play Capture the Flag with friends. To start with, you need the Explorer Kit. You get this in the house right next to the Pokémon Center in Eterna City. After you do all the stuff the old man says, you can go underground. Make a Secret Base somewhere hidden with the Digger Drill. Keep checking back to the man because he will give you things depending on what you do underground. You can also play Capture the Flag underground. If you've ever played Capture the Flag in real life, you should just about know how to play it here. Basically, everyone that is near you, on local Wi-Fi, that is also underground has a secret base too. If you manage to find someone else's base, you can steal their flag and run back to your base to claim it as your own. When you get back to your base, talk to your PC to actually steal the flag you're holding. Watch out for traps while running though, because they can cause you to end up losing your flag. If someone steals your flag, you'll see a notification above their head while running, and if you stop them by talking to them, they'll lose the flag they stole. If you or anyone else has a flag and is running, you can't talk to anyone or do anything but run without dropping your flag first.

To dig up stuff underground, find a sparkling place on your radar and tap on the screen to find a shine to dig. You can dig in the walls only. Use the Hammer or Pickaxe to dig up treasure! You can also place Spheres that you dig up in the ground and come back later to retrieve them and have them grow.

Flag Retrieval

Items to Dig Up
(Museum is in Oreburgh City)