How To Make A Secret Base In the Underground - Pokémon (BDSP)
First you need to go to Eterna City.
Click Town Map option.
You need to go underground at least once to have a chat with underground man.
The underground man wants you to dig up some treasures for him, which are invisible to the naked eye.
While you're underground, check the radar for any spots of yellow light. Go to such a location and search for treasure by pressing the A button as told by the underground man.
Go back underground and dig up a treasure from the walls.
Select "Play alone" mode to enter the grand underground!
Look for yellow glows on the map.
Select Yes! to Digg in the wall.
Start digging with a hammer.
You got a Green Sphere S!
A mismagius Statue appeared! from a pretty stone box you got after digging.
Now return to underground man to receive the digger drill.
Select 'Go Up'!
Select Yes to return to the surface from where you have entered!
You will obtain a Digger Drill from the undergroun man!
Put the digger drill in your bag's treasures pocket.
Choose 'Play alone' mode again to enter the grand underground.
Select Yes to use the digger drill to make the secret base!
Press 'A' Button on the wall you would like to destroy to make your secret base.