The Great Underground are among the few novelties brought by the remakes of the fourth generation, Pokémon Sparkling Diamond and Sparkling Pearl, and are considered as a side activity to the main adventure. It is true that you do not need to go deep into Sinnoh to complete your adventure, but there are plenty of other important reasons to spend a few hours there. Moreover, once the Pokémon League is beaten, the Great Undergrounds become even more interesting.
You can catch rare Pokémon there
The different biomes of the Great Underground have the particularity of sheltering quite rare Pokémon such as Absol, Griknot or even Togepi. It is not only very nice to see them directly in freedom but also interesting to know that it is possible to obtain the starters of the four generations or even the very useful Metamorph.
You can build your Secret Base there
Thanks to the quests of theUnderground man, you will quickly learn to mine to find precious objects but also to create your own Secret Base with the Drilling machine. Unlike the basic opus, it is then impossible to decorate it with household items such as sofas or other soft toys. On the other hand, it is the ideal place to put statues which will then give you bonus effects.
You can find great treasures there that are very useful
Of course, statues are fun and they decorate your Secret Base well, but mining in the walls of the Grands Souterrains will bring you much more than a few spheres and evolution stones: you can sometimes find much rarer items called Strange bursts. These will then be used to buy tablets allowing you to summon Legendaries in Rosa Rugosa Park.
We can get shiny more easily
While the Masuda method (eggs) and Poké Radar are currently the easiest techniques to obtain Chromatic Pokémon, you should know that the Great Undergrounds can also help you. Here, no precise method but the possibility of increasing your chances of finding a shiny thanks to the mole hunters. Indeed, once the 40 creatures have been collected (this is easier in multiplayer), a great luck bonus will activate for 4 minutes. It is notably thanks to the latter that you will be able to find luxurious chests by mining the walls.