The upcoming Pokémon Legends: Arceus promises a new and exciting adventure set in an ancient version of the familiar Sinnoh region, and the Grand Underground from Brilliant Diamond and Shining Pearl has the possibility to make an appearance. Hisui, the game's setting, is what Sinnoh was known as in times long past, which means that the game's underground areas could mimic the vast cave network that the Gen 4 games introduced. Although there's no hard evidence to support it at the moment, there is also no evidence to disprove it, which means that anything is possible.
The Grand Underground is a feature that debuted in the original Gen 4 games, Diamond and Pearl, and was given new life in the remakes. It was a major point in the trailers and reveals leading up to the game's release, and many new features were added to Pokémon BDSP's Grand Underground, such as finding Digletts. Every mainline game has featured caves, mines, or underground areas in some form, and since Pokémon Legends: Arceus seems to be focused heavily on exploration and the game's environment, it's not unreasonable to say that it could as well. Those caves have the possibility to show players what the Grand Underground looked like in Sinnoh's ancient past.
The main point of the Grand Underground is for players to obtain rare items, such as fossils, evolutionary stones, and Spheres that can be traded for items. Although it's unlikely that Pokémon Legends: Arceus will mimic it exactly, it could give it a new form and a new purpose. The caves that would one day become the Grand Underground have the potential to be a beautiful part of the game, seamlessly integrated with the rest of Hisui. The ancient Grand Underground could fix problems in Pokémon BDSP's, making it an even better experience than in the mainline games. The environments that have been shown in trailers so far already look stunning, so the region's caves could be just as much so. Its very existence may currently be pure speculation, but Hisui's Underground has the potential to be a fan-favorite portion of the game.
Pokémon Legends: Arceus Could Show Players Sinnoh's Ancient Underground
BDSP's Grand Underground is a massive series of caves that stretches across the entire Sinnoh region. Since it's such a vast cave network, it's safe to say that it at least partially formed naturally. There are modern supports and lights within the caves, and the walls are squared off into neat hallways, but the sheer magnitude of it all means that it's highly unlikely to be completely man-made, especially given the Hideaways that litter the cave network and the rare Pokémon that spawn in BDSP's Grand Underground. The unrefined, untamed version just might be one, or more, of the areas that Legends: Arceus players will get to explore.
There are two facts that are possibly the biggest arguments for the possibility of some form of the Grand Underground's inclusion in Pokémon Legends: Arceus. One is the fact that the game will not be completely open-world as fans initially expected, and will instead be broken up into different areas that will be completely separate from each other, unlike the vastness of Sword and Shield's Wild Area. The other is the fact that the Underground is split into six distinct portions, each with different biomes and environmental features. Alone, neither of these mean much, but the fact that the Grand Underground is divided in such a way that would easily allow it to become separate areas for Legends: Arceus means that the possibility is there. Since it's not open like SWSH's Wild Area, Pokémon BDSP's Grand Underground would translate well to the structure of the game's world.
If the Grand Underground does make an appearance in Pokémon Legends: Arceus, it would likely be vastly different from what it is in BDSP. Not only in terms of its aesthetics, but also in its purpose. The main goal in the Grand Underground is to obtain rare items and Pokémon that can't be found elsewhere and to collect Statues to build up the players' Secret Base. Without Secret Bases, Statues, and exclusive items and Pokémon species, the Grand Underground could become a beautiful environment, or even a series of environments, that players would enjoy exploring for hours. The details of the different areas that will be in the game are so far unknown, so some based on, or even explicitly stated to be, future areas of the Underground would likely delight players who come across them. Catching the different Pokémon in Legends: Arceus' caves and diving through their twisting tunnels can end up becoming the best part of the game for some.
How Similar Would Pokémon Legends: Arceus' Grand Underground Be To BDSP's?
The Grand Underground is unlikely to be a carbon copy if it is indeed in Legends: Arceus. The entire premise of Hisui is that it's Sinnoh as it used to be, so a more ancient and primitive version is a greater possibility. Exactly how many similarities and differences it would have - if it will even be in the game - remains to be seen, but there are a few things that it could have in common with the version that fans are familiar with. Likewise, there are also some that could be entirely different.
The similarities between the two could mostly be in their structure and environment. Pokémon Legends: Arceus has revealed some anachronisms in terms of technology, but the overworld does seem to faithfully portray its wild and primitive themes. The six areas of the Grand Underground each have different biomes and types of Hideaways, so they would fit nicely into areas that the player could explore. Since they are also only able to be unlocked by progressing the story and reaching certain towns, cities, and Routes in Sinnoh, the different sections could also be unlocked around the same areas that they are in BDSP, or at the very least their equivalents. Whether the floor plan of each area would translate to the new game, roughly or not, is impossible to predict.
The differences between BDSP's Grand Underground and a possible Hisui equivalent could boil down to human interference. The Hideaways showcase Legends: Arceus' open gameplay and world, so they could reflect the cave systems as a whole. The wooden supports, lights, and clean-cut hallways could be absent from Hisui's caves, especially since the game seems to focus so much on exploration. The themes of a more natural Sinnoh and an unrefined region could very well make themselves known in the underground caves. Winding caverns that delve deep below the surface of the earth, twisting tree roots, sparkling crystal formations, and hidden underground grottoes are more of a possibility than stone hallways. Pokémon Legends: Arceus has the opportunity to show players what the Grand Underground looked like before humans started digging for Spheres in it.