How do you get under the berry masters house on Pokemon diamond?

To access the hidden location beneath the Berry Master's house in the Pokemon Diamond version of the game, follow these steps:

Inside the Berry Master's house, locate the bookcase containing the Berry Books, which are guides for growing berries.

Interact with the bookcase and check the fifth book placed towards the left side of the shelf.

Once you check the book, a hidden button will be located behind it.

Press the hidden button to open a secret passageway that leads underground to the hidden area beneath the Berry Master's house.

You'll find yourself in the secret underground location, where you may encounter some rare and unique Pokemon or even discover hidden items and treasures.