The Grand Underground has been updated quite a bit in Brilliant Diamond and Shining Pearl. Down here, you can dig for spheres, elemental stones, and statues. Not to mention, you can run into wild Pokémon.
Some bugs in Pokémon Brilliant Diamond and Shining Pearl can cause you to get stuck in strange spots. Fortunately, you can revert your game to a previous save point. There are also a few other things to try before resetting your game.