I love the underground! Whenever I have some time, I would go down to my secret base just for fun and decorate it like what I did for my secret base in RSE. I didn't have any wifi connection though so I couldn't do the flag hunt stuff with other people, but it didn't matter because the good thing about this feature is that you don't need to have a connection in order to dig up spheres, traps or treasure. That's where all the fun's at! I'd spend some time locating, digging up and burying spheres, occasionally striking lucky on a fossil or two, some stones, heart scales, shards, or rocks. I got almost all the Fossil Pokemon using this method. I also had some fun setting traps for no one in particular because why not? Lol.