Straight from Serebii
The main thing that you do in the Sinnoh Underground is Excavating in the walls for various items. After locating them, this is controlled purely with the touch screen. It gives you a Pick Axe and a Hammer to use to try and break through the rocks in order to find 2 to 4 items. However there is a limit on how much stress you can induce on the wall through this before the wall collapses itself. To make things worse, there a good number of rocks that are just there to take up space that can cause a cave in if you keep hitting them.
Like I thought just there to take up Space and greater your chances of collapsing your cave ;( Source.
I love Weird things ¯(°_o)/¯