Here's how to find Pokémon in the Grand Underground:
1. Reach the Grand Underground: You'll unlock access to the Grand Underground after you defeat the Gym Leader in Eterna City.
2. Digging: You can dig for Pokémon using your Poké Radar and a shovel. You'll find different Pokémon depending on the type of rock you dig in.
3. Use the Poké Radar: The Poké Radar can be used to locate Pokémon by finding glowing spots in the ground.
4. Explore the Underground: There are many different areas to explore in the Grand Underground, each with its own unique Pokémon.
5. Look for Shaking Spots: Some Pokémon will be hidden in the ground and will appear as shaking spots.
6. Use the Pokédex to find out where to find Pokémon: The Pokédex will tell you what Pokémon are available in the Grand Underground and where they are likely to be found.
Here's a code example for how to get a Pokémon in the Grand Underground:
```
go_to_grand_underground()
use_poke_radar()
dig_at_glowing_spot()
catch_pokemon()
```
Remember, you can't get Pokémon from the museum. You need to explore the Grand Underground to find them.