Hello, since I never found a guide for sinnoh's underground daily quest I've decided to make one myself.
Sinnoh's underground is a special daily quest in the sense that you can get no reward, a reward room allowing you to revive a fossile for free or the secret area room that may contain hisui-voltorb, hisui-growlith, scyther holder black augurit and teddiursa holding peat block.
REQUIREMENT
A ground type with dig (dugtrio in my exemple)
A fire type with dig (arcanine in my exemple)
A rock type with dig (Lycanrock in my exemple)
A water type with dig (Blastoise in my exemple)
(if you want to access the secret area) a grass type with dig (breloom in my exemple)
A pokemon with flash or a flashlight
At least 3000 pk dollars to enter said underground
As a remeinder your dig pkm must have at least 151hapinness to dig, and the grass type with dig must be at max hapiness (255)
tips: the pokémon you will face in the underground scale relatively to your party's level, if you think your digging pkm will not fight don't level them up.
You can enter sinnoh's underground by talking to the npc in the house next to the pkm center of eterna city by paying 3k
I'll cover the secret area first.
To access the secret area you must find the Probopas who acts differently from the others. said probopass will use explosion and/or have sturdy when others don't
Each probopass 'guards' a dighole, make your grass pkm dig the hole previously guarded by the odd probopass to get the grass crystal and note down the location as you'll have to go there again at the end
Once you have the grass crystal talk to the ace trainer npc who talks about four crystals type, she'll give you the spot where to place the grass crystal.
The final puzzle solution never involves a grass crystal but you have to put it in to get the secret area to spawn
After that it's the same as for normal area except that you MUST dig where you got the grass crystal once the puzzle is solved
Normal area.
In one of the 4 rooms there's a npc archeologist that will tell you which is the most common crystal type in the solution if you defeat him in battle. (note that he disapears when beaten so pay attention to what he says. his lineup is always the same and the lvl of his pkm scales on your party's average level)
The solution to the puzzle is made of 4 types of crystal, rock, ground, water, fire. whenever you dig a hole with a pkm who's type is one of those your odds of finding crystal of that type raise (I think hapiness also plays a part in that).
The leftmost column makes a blue stone if the crystal you put in the middle is part of the solution, the rightmost column makes a blue stone if the crystal you put is in the right position. once the two columns are full you've unlocked the treasure room.
Once you've completed the puzzle you can now dig to get the tresure room (dig the spot where you got grass crystal to get to the secret area)
in the secret area you can find at random hisui-voltorb, hisui-growlith, scyther and teddiursa walking around.
the scyther and teddiursa will hold their related evolutionary items.
in the regular treasure room you will get items and the possibility to revive a fossil for free
This has been my first guide, feel free to correct me if I made any mistakes or wasen't clear enough.