This may seem very noobish. But when you go underground how can you excavate...

Step one stylus touches screen
Effect something shiny might apear on the walls go to step 2 nothing happens walk around and try again

Step two walk to where shiny thing was and press A
Effect question appears you say YES

Step 3 get either pick and hit rocks with stylus
Effect something might come out from rocks

Yes it does seem a bit noobish, but that is of no relavance, it's extremely easy. All you have to do is look on the top screen for yellow shines, when you see one, check the map in greater detail to see if it's in an available area to you, this is because certain parts of the underground are blocked off. If it's in the same area as you, run to it until your character's red dot is directly over it, then tap the touch screen. You will see several white shines, go up to one and then press A, this will bring you to an option to dig into the wall, click yes and you'll be brought into the Fossil Hunting mini-game. Oh, and one more thing to bear in mind about the shines, you will sometimes see some on the ground, these are traps, walking over one will obviously activate it, so what you have to do is go up next to it and press A, this will tell you what trap it was, and your character will disable it and automatically put it in your bag's "Traps" pocket.

Anyway, back to the mini-game itself, you will see some beige rocks normally, these are the easiest to break as one hit reveals the wall floor, and hopefully the treasures beneath, there will also always be some dark brown rocks, these are the second layer and turn into beige rocks when struck. Also, a little advanced tip, when you strike one, there is a crack in the rock nearby, when you click adjacent to the crack itself, you get the beige rocks with a crack in them. These cracks allow you to look through that beige section to see if an item, the floor, or a solid rock is underneath. Solid rocks are dark gray rocks that are randomly placed on the map, and can cause a cave-in if you keep hitting them. Finally, there is a possible third layer made up of simple rocks, no real gimmick to them except that it's difficult to get through to the bottom through all three levels and keep the stress on the wall to a minimum. There is also by the way a certain degree of stress you can inflict on the wall before it collapses naturally, basically, a simple crack at the top of the screen tells you how much more stress the wall can take, when it reaches all the way to the left side of the bar the screen will shake and the wall will collapse. And on the subject of stress inducing, now we get to your arsenal, your basic rock hammer, and pickaxe, I have found that the rock hammer is the best thing to start with, although not always.It breaks down what you want it to effectively, and is good for searching, whereas the pick only does one square, a few if you keep hitting the same spot, so it's good for finishing off whats left of the rocks surrounding an item. Regarding items, the underground has a number of useful items, too many to list here, but as an example I've gotten revives and max revives, wet stones, evolution stones, two iron balls and several Arceus plates, resulting in many of my plates being in multiple copies, just to name a few items.
As you can see, this Is the main feature of the underground, you will spend alot of time doing it, and I garuantee, it is extremely addicting, EXTREMELY, addicting. Using this method I've financed a number of expensive items for my Secret Base. I hope I have sufficiently explained the intricacies of this extremely fun mini-game, and I wish you good luck.

CrystalTitan1, A.K.A. The Q